* Will Deacon [off-list ref] [210422 09:01]:

On Tue, Apr 20, 2021 at 04:50:12PM +0000, Liam Howlett wrote:

quoted

arm64_notify_segfault() was written to decide on the si_code from the
assembly emulation of the swp_handler(), but was also used for the
signal generation from failed access_ok() and unaligned instructions.

When access_ok() fails, there is no need to search for the offending
address in the VMA space.  Instead, simply set the error to SIGSEGV with
si_code SEGV_ACCERR.

Change the return code from emulate_swpX() when there is an unaligned
pointer so the caller can differentiate from the EFAULT.  It is
unnecessary to search the VMAs in the case of an unaligned pointer.
This change uses SIGSEGV and SEGV_ACCERR instead of SIGBUS to keep with
what was returned before.

Fixes: bd35a4adc413 (arm64: Port SWP/SWPB emulation support from arm)
Signed-off-by: Liam R. Howlett <redacted>
---
 arch/arm64/kernel/armv8_deprecated.c | 20 +++++++++++++-------

Can you give an example of something that is fixed by this, please? At first
glance, it doesn't look like it changes the user-visible behaviour.

In short, when !access_ok(), don't return SEGV_MAPERR.

access_ok() is defined as __range_ok() which checks if the address is a
userspace address.  If the access is not okay, then the return should be
SEGV_ACCERR.  However, if the address is above any known VMA, then the
return will be SEGV_MAPERR.  Isn't this a bug?

Right now this particular bug is masked almost always by the fact that
find_vma() will return SIGV_ACCERR unless it's abvove any known VMA, but
patch 3 in this series will alter the behaviour and thus, I wanted to
fix the bug here before fixing that bug.

We should also be compatible with arch/arm/ here (see set_segfault()).

Yes, the same error exists there it seems.  If my solution is
acceptable, I can expand it to include the same change there.

Thanks,
Liam
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