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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11.07.24 07:13, Pei Li wrote: > This patch fixes this warning by acquiring read lock before entering > untrack_pfn() while write lock is not held. > > syzbot has tested the proposed patch and the reproducer did not > trigger any issue. > > Reported-by: syzbot+35a4414f6e247f515443@syzkaller.appspotmail.com > Closes: https://syzkaller.appspot.com/bug?extid=35a4414f6e247f515443 > Tested-by: syzbot+35a4414f6e247f515443@syzkaller.appspotmail.com > Signed-off-by: Pei Li > --- > Syzbot reported the following warning in follow_pte(): > > WARNING: CPU: 3 PID: 5192 at include/linux/rwsem.h:195 rwsem_assert_held include/linux/rwsem.h:195 [inline] > WARNING: CPU: 3 PID: 5192 at include/linux/rwsem.h:195 mmap_assert_locked include/linux/mmap_lock.h:65 [inline] > WARNING: CPU: 3 PID: 5192 at include/linux/rwsem.h:195 follow_pte+0x414/0x4c0 mm/memory.c:5980 > > This is because we are assuming that mm->mmap_lock should be held when > entering follow_pte(). This is added in commit c5541ba378e3 (mm: > follow_pte() improvements). > > However, in the following call stack, we are not acquring the lock: > follow_phys arch/x86/mm/pat/memtype.c:957 [inline] > get_pat_info+0xf2/0x510 arch/x86/mm/pat/memtype.c:991 > untrack_pfn+0xf7/0x4d0 arch/x86/mm/pat/memtype.c:1104 > unmap_single_vma+0x1bd/0x2b0 mm/memory.c:1819 > zap_page_range_single+0x326/0x560 mm/memory.c:1920 That implies that unmap_vmas() is called without the mmap lock in read mode, correct? Do we know how this happens? * exit_mmap() holds the mmap lock in read mode * unmap_region is documented to hold the mmap lock in read mode > > In zap_page_range_single(), we passed mm_wr_locked as false, as we do > not expect write lock to be held. > In the special case where vma->vm_flags is set as VM_PFNMAP, we are > hitting untrack_pfn() which eventually calls into follow_phys. > > This patch fixes this warning by acquiring read lock before entering > untrack_pfn() while write lock is not held. > > syzbot has tested the proposed patch and the reproducer did not trigger any issue: > > Tested on: > > commit: 9d9a2f29 Merge tag 'mm-hotfixes-stable-2024-07-10-13-1.. > git tree: upstream > console output: https://syzkaller.appspot.com/x/log.txt?x=13be8021980000 > kernel config: https://syzkaller.appspot.com/x/.config?x=3456bae478301dc8 > dashboard link: https://syzkaller.appspot.com/bug?extid=35a4414f6e247f515443 > compiler: gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40 > patch: https://syzkaller.appspot.com/x/patch.diff?x=145e3441980000 > > Note: testing is done by a robot and is best-effort only. > --- > mm/memory.c | 9 ++++++++- > 1 file changed, 8 insertions(+), 1 deletion(-) > > diff --git a/mm/memory.c b/mm/memory.c > index d10e616d7389..75d7959b835b 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -1815,9 +1815,16 @@ static void unmap_single_vma(struct mmu_gather *tlb, > if (vma->vm_file) > uprobe_munmap(vma, start, end); > > - if (unlikely(vma->vm_flags & VM_PFNMAP)) > + if (unlikely(vma->vm_flags & VM_PFNMAP)) { > + if (!mm_wr_locked) > + mmap_read_lock(vma->vm_mm); > + > untrack_pfn(vma, 0, 0, mm_wr_locked); > > + if (!mm_wr_locked) > + mmap_read_unlock(vma->vm_mm); > + } > + > if (start != end) { > if (unlikely(is_vm_hugetlb_page(vma))) { I'm not sure if this is the right fix. I like to understand how we end up without the mmap lock at least in read mode in that path? -- Cheers, David / dhildenb