On Fri, Dec 06, 2024 at 05:40:39PM -0500, Liam R. Howlett wrote: > * Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> [241206 16:52]: > > On second merge attempt on mmap() we incorrectly discard the possibly > > merged VMA, resulting in a possible use-after-free (and most certainly a > > reference to the wrong VMA) in this instance in the subsequent > > __mmap_complete() invocation. > > > > Correct this mistake by reassigning vma correctly if a merge succeeds in > > this case. > > > > Suggested-by: Jann Horn <jannh@xxxxxxxxxx> > > Reported-by: syzbot+91cf8da9401355f946c3@xxxxxxxxxxxxxxxxxxxxxxxxx > > Closes: https://lore.kernel.org/all/67536a25.050a0220.a30f1.0149.GAE@xxxxxxxxxx/ > > Fixes: 5ac87a885aec ("mm: defer second attempt at merge on mmap()") > > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> > > My cries to delete this optimisation continue. I'd love to too, but it does seem that has an important use in that you get a file system/driver that sets certain VMA flags. So you have: [ VMA B ] <- map [ VMA A ] < space > [ VMA C ] Merge attempt #1 - missing VM_xxx that is set by f_op->mmap() hook Merge attempt #2 - succeed because VM_xxx is set. This, I think, speaks to the need to provide a means by which filesystems/drivers can manipulate VMA flags _up front_. At which point we can eliminate this. I mean as you say this is an -optimisation- and having split VMAs will work fine (modulo mremap weirdness), but because of the above it becomes possible for there to be a major proliferation of unneeded VMAs, so it's not quite as much of a fringe thing as it might appear to be. I mean we can also talk about the fact that there's logic in __mmap_complete() that clears VM_LOCKED | VM_LOCKONFAULT under certain circumstances which is just not considered for merging at all but... yeah. That is, at least, much less likely. > > Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> Thanks :) > > > --- > > mm/vma.c | 5 ++++- > > 1 file changed, 4 insertions(+), 1 deletion(-) > > > > diff --git a/mm/vma.c b/mm/vma.c > > index 8a454a7bbc80..1ec349141f5e 100644 > > --- a/mm/vma.c > > +++ b/mm/vma.c > > @@ -2460,10 +2460,13 @@ unsigned long __mmap_region(struct file *file, unsigned long addr, > > > > /* If flags changed, we might be able to merge, so try again. */ > > if (map.retry_merge) { > > + struct vm_area_struct *merged; > > VMG_MMAP_STATE(vmg, &map, vma); > > > > vma_iter_config(map.vmi, map.addr, map.end); > > - vma_merge_existing_range(&vmg); > > + merged = vma_merge_existing_range(&vmg); > > + if (merged) > > + vma = merged; > > } > > > > __mmap_complete(&map, vma); > > -- > > 2.47.1
