Re: [PATCH] mm/mremap: populate calculated fields in vrm state correctly

[Pedro Falcato <pfalcato@suse.de>]

On Mon, Oct 19, 2026 at 03:28:27PM +0100, Lorenzo Stoakes (ARM) wrote:
> Commit d5c8aec0542e ("mm/mremap: initial refactor of move_vma()")
> introduced the vma_remap_struct helper to pass state sanely through
> mremap() logic.
> 
> Part of this state is vrm->delta - equal to the absolute difference between
> vrm->old_len and vrm->new_len (the old and new lengths of the VMA,
> respectively).
> 
> This value is used to correctly charge accounted VMAs.
> 
> When shrinking a VMA, mremap_to() sets vrm->old_len equal to vrm->new_len
> having performed the shrink of the VMA first, prior to any potential
> move (e.g. MREMAP_FIXED | MREMAP_MAYMOVE specified).
> 
> However it does not recalculate vrm->delta, which results in incorrectly
> overcharging the VMA.
> 
> The charge leaks to Committed_AS globally, meaning a mischievous user can
> cause allocations to fail across the system in strict overcommit mode.
> 
> Resolve the issue by abstracting the update of vrm->delta to
> vrm_populate_calculated() and invoking this from mremap_to() when
> vrm->old_len is changed.
> 
> Additionally, update vrm->remap_type whose value depends on
> vrm->delta. The field is not read again after the shrink, but it is the
> logically correct thing to do and protects against future changes which
> break this.
> 
> Fixes: d5c8aec0542e ("mm/mremap: initial refactor of move_vma()")
> Cc: stable@vger.kernel.org
> Reported-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
> Closes: https://lore.kernel.org/all/20261017233527.797191-1-nickolai@csail.mit.edu/
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

Reviewed-by: Pedro Falcato <pfalcato@suse.de>

As always, thank you for being so responsive in dealing with AI security
reports :))

-- 
Pedro