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| author | Dave Marchevsky <davemarchevsky@fb.com> | 2021-04-01 03:07:47 +0300 |
|---|---|---|
| committer | Alexei Starovoitov <ast@kernel.org> | 2021-04-01 23:58:07 +0300 |
| commit | 06ab134ce8ecfa5a69e850f88f81c8a4c3fa91df (patch) | |
| tree | a847d2640bbf5d0ee273b6e892684fd9c4d0964f | |
| parent | 861de02e5f3f2a104eecc5af1d248cb7bf8c5f75 (diff) | |
| download | linux-06ab134ce8ecfa5a69e850f88f81c8a4c3fa91df.tar.xz | |
bpf: Refcount task stack in bpf_get_task_stack
On x86 the struct pt_regs * grabbed by task_pt_regs() points to an
offset of task->stack. The pt_regs are later dereferenced in
__bpf_get_stack (e.g. by user_mode() check). This can cause a fault if
the task in question exits while bpf_get_task_stack is executing, as
warned by task_stack_page's comment:
* When accessing the stack of a non-current task that might exit, use
* try_get_task_stack() instead. task_stack_page will return a pointer
* that could get freed out from under you.
Taking the comment's advice and using try_get_task_stack() and
put_task_stack() to hold task->stack refcount, or bail early if it's
already 0. Incrementing stack_refcount will ensure the task's stack
sticks around while we're using its data.
I noticed this bug while testing a bpf task iter similar to
bpf_iter_task_stack in selftests, except mine grabbed user stack, and
getting intermittent crashes, which resulted in dumps like:
BUG: unable to handle page fault for address: 0000000000003fe0
\#PF: supervisor read access in kernel mode
\#PF: error_code(0x0000) - not-present page
RIP: 0010:__bpf_get_stack+0xd0/0x230
<snip...>
Call Trace:
bpf_prog_0a2be35c092cb190_get_task_stacks+0x5d/0x3ec
bpf_iter_run_prog+0x24/0x81
__task_seq_show+0x58/0x80
bpf_seq_read+0xf7/0x3d0
vfs_read+0x91/0x140
ksys_read+0x59/0xd0
do_syscall_64+0x48/0x120
entry_SYSCALL_64_after_hwframe+0x44/0xa9
Fixes: fa28dcb82a38 ("bpf: Introduce helper bpf_get_task_stack()")
Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20210401000747.3648767-1-davemarchevsky@fb.com
| -rw-r--r-- | kernel/bpf/stackmap.c | 12 |
1 files changed, 10 insertions, 2 deletions
diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c index be35bfb7fb13..6fbc2abe9c91 100644 --- a/kernel/bpf/stackmap.c +++ b/kernel/bpf/stackmap.c @@ -517,9 +517,17 @@ const struct bpf_func_proto bpf_get_stack_proto = { BPF_CALL_4(bpf_get_task_stack, struct task_struct *, task, void *, buf, u32, size, u64, flags) { - struct pt_regs *regs = task_pt_regs(task); + struct pt_regs *regs; + long res; - return __bpf_get_stack(regs, task, NULL, buf, size, flags); + if (!try_get_task_stack(task)) + return -EFAULT; + + regs = task_pt_regs(task); + res = __bpf_get_stack(regs, task, NULL, buf, size, flags); + put_task_stack(task); + + return res; } BTF_ID_LIST_SINGLE(bpf_get_task_stack_btf_ids, struct, task_struct) |