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Laurent reported that STRICT_MODULE_RWX was causing intermittent crashes
on one of his systems:
kernel tried to execute exec-protected page (c008000004073278) - exploit attempt? (uid: 0)
BUG: Unable to handle kernel instruction fetch
Faulting instruction address: 0xc008000004073278
Oops: Kernel access of bad area, sig: 11 [#1]
LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=2048 NUMA pSeries
Modules linked in: drm virtio_console fuse drm_panel_orientation_quirks ...
CPU: 3 PID: 44 Comm: kworker/3:1 Not tainted 5.14.0-rc4+ #12
Workqueue: events control_work_handler [virtio_console]
NIP: c008000004073278 LR: c008000004073278 CTR: c0000000001e9de0
REGS: c00000002e4ef7e0 TRAP: 0400 Not tainted (5.14.0-rc4+)
MSR: 800000004280b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE> CR: 24002822 XER: 200400cf
...
NIP fill_queue+0xf0/0x210 [virtio_console]
LR fill_queue+0xf0/0x210 [virtio_console]
Call Trace:
fill_queue+0xb4/0x210 [virtio_console] (unreliable)
add_port+0x1a8/0x470 [virtio_console]
control_work_handler+0xbc/0x1e8 [virtio_console]
process_one_work+0x290/0x590
worker_thread+0x88/0x620
kthread+0x194/0x1a0
ret_from_kernel_thread+0x5c/0x64
Jordan, Fabiano & Murilo were able to reproduce and identify that the
problem is caused by the call to module_enable_ro() in do_init_module(),
which happens after the module's init function has already been called.
Our current implementation of change_page_attr() is not safe against
concurrent accesses, because it invalidates the PTE before flushing the
TLB and then installing the new PTE. That leaves a window in time where
there is no valid PTE for the page, if another CPU tries to access the
page at that time we see something like the fault above.
We can't simply switch to set_pte_at()/flush TLB, because our hash MMU
code doesn't handle a set_pte_at() of a valid PTE. See [1].
But we do have pte_update(), which replaces the old PTE with the new,
meaning there's no window where the PTE is invalid. And the hash MMU
version hash__pte_update() deals with synchronising the hash page table
correctly.
[1]: https://lore.kernel.org/linuxppc-dev/87y318wp9r.fsf@linux.ibm.com/
Fixes: 1f9ad21c3b38 ("powerpc/mm: Implement set_memory() routines")
Reported-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Murilo Opsfelder Araújo <muriloo@linux.ibm.com>
Tested-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210818120518.3603172-1-mpe@ellerman.id.au
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The set_memory_{ro/rw/nx/x}() functions are required for
STRICT_MODULE_RWX, and are generally useful primitives to have. This
implementation is designed to be generic across powerpc's many MMUs.
It's possible that this could be optimised to be faster for specific
MMUs.
This implementation does not handle cases where the caller is attempting
to change the mapping of the page it is executing from, or if another
CPU is concurrently using the page being altered. These cases likely
shouldn't happen, but a more complex implementation with MMU-specific code
could safely handle them.
On hash, the linear mapping is not kept in the linux pagetable, so this
will not change the protection if used on that range. Currently these
functions are not used on the linear map so just WARN for now.
apply_to_existing_page_range() does not work on huge pages so for now
disallow changing the protection of huge pages.
[jpn: - Allow set memory functions to be used without Strict RWX
- Hash: Disallow certain regions
- Have change_page_attr() take function pointers to manipulate ptes
- Radix: Add ptesync after set_pte_at()]
Signed-off-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210609013431.9805-2-jniethe5@gmail.com
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