On Tue, Nov 19, 2024 at 12:59:45PM -0500, Liam R. Howlett wrote: > From: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx> > > The mmap_region() function tries to install a new vma, which requires a > pre-allocation for the maple tree write due to the complex locking > scenarios involved. > > Recent efforts to simplify the error recovery required the relocation of > the preallocation of the maple tree nodes (via vma_iter_prealloc() > calling mas_preallocate()) higher in the function. > > The relocation of the preallocation meant that, if there was a file > associated with the vma and the driver call (mmap_file()) modified the > vma flags, then a new merge of the new vma with existing vmas is > attempted. > > During the attempt to merge the existing vma with the new vma, the vma > iterator is used - the same iterator that would be used for the next > write attempt to the tree. In the event of needing a further allocation > and if the new allocations fails, the vma iterator (and contained maple > state) will cleaned up, including freeing all previous allocations and > will be reset internally. > > Upon returning to the __mmap_region() function, the error is available > in the vma_merge_struct and can be used to detect the -ENOMEM status. > > Hitting an -ENOMEM scenario after the driver callback leaves the system > in a state that undoing the mapping is worse than continuing by dipping > into the reserve. > > A preallocation should be performed in the case of an -ENOMEM and the > allocations were lost during the failure scenario. The __GFP_NOFAIL > flag is used in the allocation to ensure the allocation succeeds after > implicitly telling the driver that the mapping was happening. > > The range is already set in the vma_iter_store() call below, so it is > not necessary and is dropped. > > Reported-by: syzbot+bc6bfc25a68b7a020ee1@xxxxxxxxxxxxxxxxxxxxxxxxx > Closes: https://syzkaller.appspot.com/x/log.txt?x=17b0ace8580000 > Fixes: 5de195060b2e2 ("mm: resolve faulty mmap_region() error path behaviour") > Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> > Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> > Cc: Vlastimil Babka <vbabka@xxxxxxx> > Cc: Jann Horn <jannh@xxxxxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> Looks good to me: Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> I mean ideally we'd not have to handle this scenario, but 6.13 resolves this in a different way, and since this will never nearly happen (perhaps actually never in reality), I think having an operation that will nearly always be a no-op beats out alternative solutions. > --- > mm/mmap.c | 13 ++++++++++++- > 1 file changed, 12 insertions(+), 1 deletion(-) > > Changes since v1: > - Don't bail out and force the allocation when the merge failure is > -ENOMEM - Thanks Lorenzo > > diff --git a/mm/mmap.c b/mm/mmap.c > index 79d541f1502b2..4f6e566d52faa 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -1491,7 +1491,18 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr, > vm_flags = vma->vm_flags; > goto file_expanded; > } > - vma_iter_config(&vmi, addr, end); > + > + /* > + * In the unlikely even that more memory was needed, but > + * not available for the vma merge, the vma iterator > + * will have no memory reserved for the write we told > + * the driver was happening. To keep up the ruse, > + * ensure the allocation for the store succeeds. > + */ > + if (vmg_nomem(&vmg)) { > + mas_preallocate(&vmi.mas, vma, > + GFP_KERNEL|__GFP_NOFAIL); > + } > } > > vm_flags = vma->vm_flags; > -- > 2.43.0 >
