core: fix race in mobj_reg_shm_dec_map() #6137
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Another fix for #6120 |
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Fixes a race in mobj_reg_shm_dec_map() when r->mm is NULL. This is similar to the race fixed by commit 06ea466 ("core: fix race in mobj_reg_shm_inc_map()"), but with one more possibility. The problem goes like: A. Thread 1 calls mobj_reg_shm_dec_map() at the same time as thread 2 calls mobj_reg_shm_inc_map(). B. Thread 1 decreases mapcount to zero and tries to take the spinlock, but thread 1 is suspended before it has acquired the spinlock. C. Thread 2 sees that mapcount is zero and takes the spinlock and maps the memory. D. Thread 2 calls mobj_reg_shm_dec_map(), mapcount reaches zero again and the shared memory is unmapped and r->mm is set to NULL. E. Thread 1 is finally resumed and acquires the spinlock, mapcount is still zero but r->mm is also NULL. To fix the problem at step E above check that r->mm is still non-NULL. Note that the same fix isn't needed for ffa_dec_map() since unmap_helper() checks that mf->mm is non-NULL first. Fixes: 06ea466 ("core: fix race in mobj_reg_shm_inc_map()") Signed-off-by: Jens Wiklander <[email protected]> Reviewed-by: Jerome Forissier <[email protected]> Acked-by: Olivier Masse <[email protected]>
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Fixes a race in mobj_reg_shm_dec_map() when r->mm is NULL. This is similar to the race fixed by commit 06ea466 ("core: fix race in mobj_reg_shm_inc_map()"), but with one more possibility.
The problem goes like:
A. Thread 1 calls mobj_reg_shm_dec_map() at the same time as thread 2
calls mobj_reg_shm_inc_map().
B. Thread 1 decreases mapcount to zero and tries to take the spinlock,
but thread 1 is suspended before it has acquired the spinlock.
C. Thread 2 sees that mapcount is zero and takes the spinlock and maps
the memory.
D. Thread 2 calls mobj_reg_shm_dec_map(), mapcount reaches zero again
and the shared memory is unmapped and r->mm is set to NULL.
E. Thread 1 is finally resumed and acquires the spinlock, mapcount is still
zero but r->mm is also NULL.
To fix the problem at step E above check that r->mm is still non-NULL.
Fixes: 06ea466 ("core: fix race in mobj_reg_shm_inc_map()")