kernel/linux.git/include/linux/sched.h, branch v6.12.107

kcov: fix data corruption and race conditions on PREEMPT_RT

2026-08-27T12:32:05+00:00

commit 2eed77fdcb0cc48e8eccb2bcd4b7f2c6d650e84c upstream. syzbot is reporting KCOV state corruption on PREEMPT_RT kernels, for the temporary storage used for saving/restoring remote KCOV state is currently allocated as the per-CPU area. On PREEMPT_RT kernels, softirq handlers run as preemptible task threads (e.g., ksoftirqd). If a softirq context preempts a task running a remote KCOV session, it safely saves the task's state into the per-CPU area. However, if that softirq thread is subsequently preempted by a higher- priority softirq thread on the same CPU, the second softirq will overwrite the same per-CPU area, permanently destroying the original task's KCOV state. Fix this data corruption by moving the temporary storage from the per-CPU area to the per-thread area. Since each softirq thread now owns its own task context, nested softirq preemption no longer causes data overwrites. Note that while the temporary storage is now on a per-thread basis, the per-CPU kcov_percpu_data.lock must be retained, for we need to ensure that kcov_remote_start() and kcov_remote_stop() operate atomically without racing against asynchronous interrupts that manipulate the current task's KCOV state. It is likely that GFP_KERNEL allocation by vmalloc_node() in kcov_init() has already called panic() before returning NULL, for there will be no OOM-killable userspace processes when __init function of built-in module runs. But this patch also fixes crashing the kernel when vmalloc_node() in kcov_init() returned NULL, for kcov_init() left per-CPU irq_area == NULL but kcov_remote_start() depends on per-CPU irq_area != NULL, resulting in (1) doing vmalloc() in kcov_remote_start() despite !in_task() context (2) out-of-array-bounds access if (1) succeeded but kcov->remote_size < CONFIG_KCOV_IRQ_AREA_SIZE (3) always leak memory allocated by (1), eventually killing all OOM-killable userspace processes problems. Link: https://lore.kernel.org/43552d09-2ce2-4b19-b0d3-a2d1ab952145@I-love.SAKURA.ne.jp Reported-by: syzbot+3f51ad7ac3ae57a6fdcc@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=3f51ad7ac3ae57a6fdcc Reported-by: syzbot+47cf95ca1f9dcca872c8@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=47cf95ca1f9dcca872c8 Reported-by: syzbot+8a173e13208949931dc7@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=8a173e13208949931dc7 Reported-by: syzbot+90984d3713722683112e@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=90984d3713722683112e Analyzed-by: AI Mode in Google Search (no mail address) Fixes: 5ff3b30ab57d ("kcov: collect coverage from interrupts") Signed-off-by: Tetsuo Handa Reviewed-by: Alexander Potapenko Cc: Alan Stern Cc: Andrey Konovalov Cc: Christoph Hellwig Cc: Clark Williams Cc: Dmitry Vyukov Cc: Greg Kroah-Hartman Cc: Marco Elver Cc: Mark Brown Cc: Roman Gushchin Cc: Sebastian Andrzej Siewior Cc: Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman

sched/deadline: Fix dl_server behaviour

2026-06-01T15:46:14+00:00

commit a3a70caf7906708bf9bbc80018752a6b36543808 upstream. John reported undesirable behaviour with the dl_server since commit: cccb45d7c4295 ("sched/deadline: Less agressive dl_server handling"). When starving fair tasks on purpose (starting spinning FIFO tasks), his fair workload, which often goes (briefly) idle, would delay fair invocations for a second, running one invocation per second was both unexpected and terribly slow. The reason this happens is that when dl_se->server_pick_task() returns NULL, indicating no runnable tasks, it would yield, pushing any later jobs out a whole period (1 second). Instead simply stop the server. This should restore behaviour in that a later wakeup (which restarts the server) will be able to continue running (subject to the CBS wakeup rules). Notably, this does not re-introduce the behaviour cccb45d7c4295 set out to solve, any start/stop cycle is naturally throttled by the timer period (no active cancel). Fixes: cccb45d7c4295 ("sched/deadline: Less agressive dl_server handling") Reported-by: John Stultz Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Peter Zijlstra (Intel) Tested-by: John Stultz Closes: https://lore.kernel.org/regressions/04657838-46d1-432d-95e1-eb73b930b032@mailbox.org Signed-off-by: Lukas Beckmann Signed-off-by: Sasha Levin

sched/deadline: Fix dl_server getting stuck

2026-06-01T15:46:14+00:00

commit 4ae8d9aa9f9dc7137ea5e564d79c5aa5af1bc45c upstream. John found it was easy to hit lockup warnings when running locktorture on a 2 CPU VM, which he bisected down to: commit cccb45d7c429 ("sched/deadline: Less agressive dl_server handling"). While debugging it seems there is a chance where we end up with the dl_server dequeued, with dl_se->dl_server_active. This causes dl_server_start() to return without enqueueing the dl_server, thus it fails to run when RT tasks starve the cpu. When this happens, dl_server_timer() catches the '!dl_se->server_has_tasks(dl_se)' case, which then calls replenish_dl_entity() and dl_server_stopped() and finally return HRTIMER_NO_RESTART. This ends in no new timer and also no enqueue, leaving the dl_server 'dead', allowing starvation. What should have happened is for the bandwidth timer to start the zero-laxity timer, which in turn would enqueue the dl_server and cause dl_se->server_pick_task() to be called -- which will stop the dl_server if no fair tasks are observed for a whole period. IOW, it is totally irrelevant if there are fair tasks at the moment of bandwidth refresh. This removes all dl_se->server_has_tasks() users, so remove the whole thing. Fixes: cccb45d7c4295 ("sched/deadline: Less agressive dl_server handling") Reported-by: John Stultz Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Peter Zijlstra (Intel) Tested-by: John Stultz [ adjust renamed variable in fair_server_has_tasks (which this patch removes) ] Signed-off-by: Lukas Beckmann Signed-off-by: Sasha Levin

sched/deadline: Less agressive dl_server handling

2026-06-01T15:46:14+00:00

commit cccb45d7c4295bbfeba616582d0249f2d21e6df5 upstream. Chris reported that commit 5f6bd380c7bd ("sched/rt: Remove default bandwidth control") caused a significant dip in his favourite benchmark of the day. Simply disabling dl_server cured things. His workload hammers the 0->1, 1->0 transitions, and the dl_server_{start,stop}() overhead kills it -- fairly obviously a bad idea in hind sight and all that. Change things around to only disable the dl_server when there has not been a fair task around for a whole period. Since the default period is 1 second, this ensures the benchmark never trips this, overhead gone. Fixes: 557a6bfc662c ("sched/fair: Add trivial fair server") Reported-by: Chris Mason Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Juri Lelli Acked-by: Juri Lelli Link: https://lkml.kernel.org/r/20250702121158.465086194@infradead.org [ adjust context for renamed/removed variable names ] Signed-off-by: Lukas Beckmann Signed-off-by: Sasha Levin

ptrace: slightly saner 'get_dumpable()' logic

2026-05-15T12:51:08+00:00

commit 31e62c2ebbfdc3fe3dbdf5e02c92a9dc67087a3a upstream. The 'dumpability' of a task is fundamentally about the memory image of the task - the concept comes from whether it can core dump or not - and makes no sense when you don't have an associated mm. And almost all users do in fact use it only for the case where the task has a mm pointer. But we have one odd special case: ptrace_may_access() uses 'dumpable' to check various other things entirely independently of the MM (typically explicitly using flags like PTRACE_MODE_READ_FSCREDS). Including for threads that no longer have a VM (and maybe never did, like most kernel threads). It's not what this flag was designed for, but it is what it is. The ptrace code does check that the uid/gid matches, so you do have to be uid-0 to see kernel thread details, but this means that the traditional "drop capabilities" model doesn't make any difference for this all. Make it all make a *bit* more sense by saying that if you don't have a MM pointer, we'll use a cached "last dumpability" flag if the thread ever had a MM (it will be zero for kernel threads since it is never set), and require a proper CAP_SYS_PTRACE capability to override. Reported-by: Qualys Security Advisory Cc: Oleg Nesterov Cc: Kees Cook Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman

randomize_kstack: Maintain kstack_offset per task

2026-05-07T04:09:35+00:00

commit 37beb42560165869838e7d91724f3e629db64129 upstream. kstack_offset was previously maintained per-cpu, but this caused a couple of issues. So let's instead make it per-task. Issue 1: add_random_kstack_offset() and choose_random_kstack_offset() expected and required to be called with interrupts and preemption disabled so that it could manipulate per-cpu state. But arm64, loongarch and risc-v are calling them with interrupts and preemption enabled. I don't _think_ this causes any functional issues, but it's certainly unexpected and could lead to manipulating the wrong cpu's state, which could cause a minor performance degradation due to bouncing the cache lines. By maintaining the state per-task those functions can safely be called in preemptible context. Issue 2: add_random_kstack_offset() is called before executing the syscall and expands the stack using a previously chosen random offset. choose_random_kstack_offset() is called after executing the syscall and chooses and stores a new random offset for the next syscall. With per-cpu storage for this offset, an attacker could force cpu migration during the execution of the syscall and prevent the offset from being updated for the original cpu such that it is predictable for the next syscall on that cpu. By maintaining the state per-task, this problem goes away because the per-task random offset is updated after the syscall regardless of which cpu it is executing on. Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall") Closes: https://lore.kernel.org/all/dd8c37bc-795f-4c7a-9086-69e584d8ab24@arm.com/ Cc: stable@vger.kernel.org Acked-by: Mark Rutland Signed-off-by: Ryan Roberts Link: https://patch.msgid.link/20260303150840.3789438-2-ryan.roberts@arm.com Signed-off-by: Kees Cook Signed-off-by: Greg Kroah-Hartman

perf: sched: Fix perf crash with new is_user_task() helper

2026-02-06T15:55:50+00:00

[ Upstream commit 76ed27608f7dd235b727ebbb12163438c2fbb617 ] In order to do a user space stacktrace the current task needs to be a user task that has executed in user space. It use to be possible to test if a task is a user task or not by simply checking the task_struct mm field. If it was non NULL, it was a user task and if not it was a kernel task. But things have changed over time, and some kernel tasks now have their own mm field. An idea was made to instead test PF_KTHREAD and two functions were used to wrap this check in case it became more complex to test if a task was a user task or not[1]. But this was rejected and the C code simply checked the PF_KTHREAD directly. It was later found that not all kernel threads set PF_KTHREAD. The io-uring helpers instead set PF_USER_WORKER and this needed to be added as well. But checking the flags is still not enough. There's a very small window when a task exits that it frees its mm field and it is set back to NULL. If perf were to trigger at this moment, the flags test would say its a user space task but when perf would read the mm field it would crash with at NULL pointer dereference. Now there are flags that can be used to test if a task is exiting, but they are set in areas that perf may still want to profile the user space task (to see where it exited). The only real test is to check both the flags and the mm field. Instead of making this modification in every location, create a new is_user_task() helper function that does all the tests needed to know if it is safe to read the user space memory or not. [1] https://lore.kernel.org/all/20250425204120.639530125@goodmis.org/ Fixes: 90942f9fac05 ("perf: Use current->flags & PF_KTHREAD|PF_USER_WORKER instead of current->mm == NULL") Closes: https://lore.kernel.org/all/0d877e6f-41a7-4724-875d-0b0a27b8a545@roeck-us.net/ Reported-by: Guenter Roeck Signed-off-by: Steven Rostedt (Google) Signed-off-by: Peter Zijlstra (Intel) Tested-by: Guenter Roeck Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20260129102821.46484722@gandalf.local.home Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman

NFS: fix nfs_release_folio() to not deadlock via kcompactd writeback

2025-03-13T12:01:58+00:00

commit ce6d9c1c2b5cc785016faa11b48b6cd317eb367e upstream. Add PF_KCOMPACTD flag and current_is_kcompactd() helper to check for it so nfs_release_folio() can skip calling nfs_wb_folio() from kcompactd. Otherwise NFS can deadlock waiting for kcompactd enduced writeback which recurses back to NFS (which triggers writeback to NFSD via NFS loopback mount on the same host, NFSD blocks waiting for XFS's call to __filemap_get_folio): 6070.550357] INFO: task kcompactd0:58 blocked for more than 4435 seconds. {--- [58] "kcompactd0" [<0>] folio_wait_bit+0xe8/0x200 [<0>] folio_wait_writeback+0x2b/0x80 [<0>] nfs_wb_folio+0x80/0x1b0 [nfs] [<0>] nfs_release_folio+0x68/0x130 [nfs] [<0>] split_huge_page_to_list_to_order+0x362/0x840 [<0>] migrate_pages_batch+0x43d/0xb90 [<0>] migrate_pages_sync+0x9a/0x240 [<0>] migrate_pages+0x93c/0x9f0 [<0>] compact_zone+0x8e2/0x1030 [<0>] compact_node+0xdb/0x120 [<0>] kcompactd+0x121/0x2e0 [<0>] kthread+0xcf/0x100 [<0>] ret_from_fork+0x31/0x40 [<0>] ret_from_fork_asm+0x1a/0x30 ---} [akpm@linux-foundation.org: fix build] Link: https://lkml.kernel.org/r/20250225022002.26141-1-snitzer@kernel.org Fixes: 96780ca55e3c ("NFS: fix up nfs_release_folio() to try to release the page") Signed-off-by: Mike Snitzer Cc: Anna Schumaker Cc: Trond Myklebust Cc: Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman

sched/fair: Fix value reported by hot tasks pulled in /proc/schedstat

2025-02-08T08:56:54+00:00

[ Upstream commit a430d99e349026d53e2557b7b22bd2ebd61fe12a ] In /proc/schedstat, lb_hot_gained reports the number hot tasks pulled during load balance. This value is incremented in can_migrate_task() if the task is migratable and hot. After incrementing the value, load balancer can still decide not to migrate this task leading to wrong accounting. Fix this by incrementing stats when hot tasks are detached. This issue only exists in detach_tasks() where we can decide to not migrate hot task even if it is migratable. However, in detach_one_task(), we migrate it unconditionally. [Swapnil: Handled the case where nr_failed_migrations_hot was not accounted properly and wrote commit log] Fixes: d31980846f96 ("sched: Move up affinity check to mitigate useless redoing overhead") Signed-off-by: Peter Zijlstra (Intel) Reported-by: "Gautham R. Shenoy" Not-yet-signed-off-by: Peter Zijlstra Signed-off-by: Swapnil Sapkal Signed-off-by: Peter Zijlstra (Intel) Link: https://lore.kernel.org/r/20241220063224.17767-2-swapnil.sapkal@amd.com Signed-off-by: Sasha Levin

freezer, sched: Report frozen tasks as 'D' instead of 'R'

2025-01-02T09:34:21+00:00

[ Upstream commit f718faf3940e95d5d34af9041f279f598396ab7d ] Before commit: f5d39b020809 ("freezer,sched: Rewrite core freezer logic") the frozen task stat was reported as 'D' in cgroup v1. However, after rewriting the core freezer logic, the frozen task stat is reported as 'R'. This is confusing, especially when a task with stat of 'S' is frozen. This bug can be reproduced with these steps: $ cd /sys/fs/cgroup/freezer/ $ mkdir test $ sleep 1000 & [1] 739 // task whose stat is 'S' $ echo 739 > test/cgroup.procs $ echo FROZEN > test/freezer.state $ ps -aux | grep 739 root 739 0.1 0.0 8376 1812 pts/0 R 10:56 0:00 sleep 1000 As shown above, a task whose stat is 'S' was changed to 'R' when it was frozen. To solve this regression, simply maintain the same reported state as before the rewrite. [ mingo: Enhanced the changelog and comments ] Fixes: f5d39b020809 ("freezer,sched: Rewrite core freezer logic") Signed-off-by: Chen Ridong Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Ingo Molnar Acked-by: Tejun Heo Acked-by: Michal Koutný Link: https://lore.kernel.org/r/20241217004818.3200515-1-chenridong@huaweicloud.com Signed-off-by: Sasha Levin