Message-ID: <717f1f43-3ec0-41a7-b0a9-05383a4325d4@suse.cz>
Date: Tue, 29 Jul 2025 14:03:15 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: akpm@...ux-foundation.org, jannh@...gle.com, Liam.Howlett@...cle.com,
 lorenzo.stoakes@...cle.com, pfalcato@...e.de, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/1] mm: fix a UAF when vma->mm is freed after
 vma->vm_refcnt got dropped

On 7/29/25 00:53, Suren Baghdasaryan wrote:
> On Mon, Jul 28, 2025 at 10:04 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>>
>> As for further steps, considered replying to [1] but maybe it's better here.
>>
>> As for a KISS fix including stable, great. Seems a nice improvement to
>> actually handle "vma->vm_mm != mm" in vma_start_read() like this - good idea!
>>
>> Less great is that there's now a subtle assumption that some (but not all!)
>> cases where vma_start_read() returns NULL imply that we dropped the rcu
>> lock. And it doesn't matter as the callers abort or fallback to mmap sem
>> anyway in that case. Hopefully we can improve that a bit.
>>
>> The idea of moving rcu lock and mas walk inside vma_start_read() is indeed
>> busted with lock_next_vma(). The iterator difference could be perhaps solved
>> by having lock_vma_under_rcu() set up its own one (instead of MA_STATE) in a
>> way that vma_next() would do the right thing for it. However there would
>> still be the difference that lock_next_vma() expects we are already under
>> rcu lock, and lock_vma_under_rcu() doesn't.
>>
>> So what we can perhaps do instead is move vma_start_read() to mm/mmap_lock.c
>> (no other users so why expose it in a header for potential misuse). And then
>> indeed just make it drop rcu lock completely (and not reacquire) any time
>> it's returning NULL, document that and adjust callers to that. I think it's
>> less subtle than dropping and reacquring, and should simplify the error
>> handling in the callers a bit.
> 
> Thanks for the suggestion. That was actually exactly one of the
> options I was considering but I thought this dropping RCU schema would
> still be uglier than dropping and reacquiring the RCU lock. If you
> think otherwise I can make the change as you suggested for mm-unstable
> and keep this original change for stable only. Should I do that?

If we decide anything, I would do it as a cleanup on top of the fix that
will now go to mainline and then stable. We don't want to deviate for stable
unnecessarily (removing an extraneous hunk in stable backport is fine).

As for which case is uglier, I don't know really. Dropping and reacquiring
rcy lock in very rare cases, leading to even rarer situations where it would
cause an issue, seems more dangerous to me than just dropping everytime we
return NULL for any of the reasons, which is hopefully less rare and an
error such as caller trying to drop rcu lock again will blow up immediately.
Maybe others have opinions...

>>
>> [1]
>> https://lore.kernel.org/all/CAJuCfpEMhGe_eZuFm__4CDstM9%3DOG2kTUTziNL-f%3DM3BYQor2A@mail.gmail.com/
>>
>> > ---
>> > Changes since v1 [1]
>> > - Made a copy of vma->mm before using it in vma_start_read(),
>> > per Vlastimil Babka
>> >
>> > Notes:
>> > - Applies cleanly over mm-unstable.
>> > - Should be applied to 6.15 and 6.16 but these branches do not
>> > have lock_next_vma() function, so the change in lock_next_vma() should be
>> > skipped when applying to those branches.
>> >
>> > [1] https://lore.kernel.org/all/20250728170950.2216966-1-surenb@google.com/
>> >
>> >  include/linux/mmap_lock.h | 23 +++++++++++++++++++++++
>> >  mm/mmap_lock.c            | 10 +++-------
>> >  2 files changed, 26 insertions(+), 7 deletions(-)
>> >
>> > diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
>> > index 1f4f44951abe..da34afa2f8ef 100644
>> > --- a/include/linux/mmap_lock.h
>> > +++ b/include/linux/mmap_lock.h
>> > @@ -12,6 +12,7 @@ extern int rcuwait_wake_up(struct rcuwait *w);
>> >  #include <linux/tracepoint-defs.h>
>> >  #include <linux/types.h>
>> >  #include <linux/cleanup.h>
>> > +#include <linux/sched/mm.h>
>> >
>> >  #define MMAP_LOCK_INITIALIZER(name) \
>> >       .mmap_lock = __RWSEM_INITIALIZER((name).mmap_lock),
>> > @@ -183,6 +184,28 @@ static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm,
>> >       }
>> >
>> >       rwsem_acquire_read(&vma->vmlock_dep_map, 0, 1, _RET_IP_);
>> > +
>> > +     /*
>> > +      * If vma got attached to another mm from under us, that mm is not
>> > +      * stable and can be freed in the narrow window after vma->vm_refcnt
>> > +      * is dropped and before rcuwait_wake_up(mm) is called. Grab it before
>> > +      * releasing vma->vm_refcnt.
>> > +      */
>> > +     if (unlikely(vma->vm_mm != mm)) {
>> > +             /* Use a copy of vm_mm in case vma is freed after we drop vm_refcnt */
>> > +             struct mm_struct *other_mm = vma->vm_mm;
>> > +             /*
>> > +              * __mmdrop() is a heavy operation and we don't need RCU
>> > +              * protection here. Release RCU lock during these operations.
>> > +              */
>> > +             rcu_read_unlock();
>> > +             mmgrab(other_mm);
>> > +             vma_refcount_put(vma);
>> > +             mmdrop(other_mm);
>> > +             rcu_read_lock();
>> > +             return NULL;
>> > +     }
>> > +
>> >       /*
>> >        * Overflow of vm_lock_seq/mm_lock_seq might produce false locked result.
>> >        * False unlocked result is impossible because we modify and check
>> > diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
>> > index 729fb7d0dd59..aa3bc42ecde0 100644
>> > --- a/mm/mmap_lock.c
>> > +++ b/mm/mmap_lock.c
>> > @@ -164,8 +164,7 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
>> >        */
>> >
>> >       /* Check if the vma we locked is the right one. */
>> > -     if (unlikely(vma->vm_mm != mm ||
>> > -                  address < vma->vm_start || address >= vma->vm_end))
>> > +     if (unlikely(address < vma->vm_start || address >= vma->vm_end))
>> >               goto inval_end_read;
>> >
>> >       rcu_read_unlock();
>> > @@ -236,11 +235,8 @@ struct vm_area_struct *lock_next_vma(struct mm_struct *mm,
>> >               goto fallback;
>> >       }
>> >
>> > -     /*
>> > -      * Verify the vma we locked belongs to the same address space and it's
>> > -      * not behind of the last search position.
>> > -      */
>> > -     if (unlikely(vma->vm_mm != mm || from_addr >= vma->vm_end))
>> > +     /* Verify the vma is not behind of the last search position. */
>> > +     if (unlikely(from_addr >= vma->vm_end))
>> >               goto fallback_unlock;
>> >
>> >       /*
>> >
>> > base-commit: c617a4dd7102e691fa0fb2bc4f6b369e37d7f509
>>

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