mm: lock_vma_under_rcu() must check vma->anon_vma under vma lock
lock_vma_under_rcu() tries to guarantee that __anon_vma_prepare() can't
be called in the VMA-locked page fault path by ensuring that
vma->anon_vma is set.
However, this check happens before the VMA is locked, which means a
concurrent move_vma() can concurrently call unlink_anon_vmas(), which
disassociates the VMA's anon_vma.
This means we can get UAF in the following scenario:
THREAD 1 THREAD 2
======== ========
<page fault>
lock_vma_under_rcu()
rcu_read_lock()
mas_walk()
check vma->anon_vma
mremap() syscall
move_vma()
vma_start_write()
unlink_anon_vmas()
<syscall end>
handle_mm_fault()
__handle_mm_fault()
handle_pte_fault()
do_pte_missing()
do_anonymous_page()
anon_vma_prepare()
__anon_vma_prepare()
find_mergeable_anon_vma()
mas_walk() [looks up VMA X]
munmap() syscall (deletes VMA X)
reusable_anon_vma() [called on freed VMA X]
This is a security bug if you can hit it, although an attacker would
have to win two races at once where the first race window is only a few
instructions wide.
This patch is based on some previous discussion with Linus Torvalds on
the security list.
Cc: stable@vger.kernel.org
Fixes: 5e31275cc9
("mm: add per-VMA lock and helper functions to control it")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
parent
0a8db05b57
commit
657b514695
28
mm/memory.c
28
mm/memory.c
@ -5393,27 +5393,28 @@ retry:
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if (!vma_is_anonymous(vma) && !vma_is_tcp(vma))
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goto inval;
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/* find_mergeable_anon_vma uses adjacent vmas which are not locked */
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if (!vma->anon_vma && !vma_is_tcp(vma))
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goto inval;
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if (!vma_start_read(vma))
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goto inval;
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/*
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* find_mergeable_anon_vma uses adjacent vmas which are not locked.
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* This check must happen after vma_start_read(); otherwise, a
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* concurrent mremap() with MREMAP_DONTUNMAP could dissociate the VMA
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* from its anon_vma.
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*/
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if (unlikely(!vma->anon_vma && !vma_is_tcp(vma)))
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goto inval_end_read;
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/*
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* Due to the possibility of userfault handler dropping mmap_lock, avoid
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* it for now and fall back to page fault handling under mmap_lock.
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*/
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if (userfaultfd_armed(vma)) {
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vma_end_read(vma);
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goto inval;
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}
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if (userfaultfd_armed(vma))
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goto inval_end_read;
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/* Check since vm_start/vm_end might change before we lock the VMA */
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if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
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vma_end_read(vma);
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goto inval;
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}
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if (unlikely(address < vma->vm_start || address >= vma->vm_end))
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goto inval_end_read;
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/* Check if the VMA got isolated after we found it */
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if (vma->detached) {
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@ -5425,6 +5426,9 @@ retry:
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rcu_read_unlock();
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return vma;
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inval_end_read:
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vma_end_read(vma);
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inval:
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rcu_read_unlock();
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count_vm_vma_lock_event(VMA_LOCK_ABORT);
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