On 3/21/22 11:25, syzbot wrote:
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=12a2d0a9700000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=13d34fd9700000
The issue was bisected to:
commit ed922739c9199bf515a3e7fec3e319ce1edeef2a
Author: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>
Date: Mon Dec 6 19:54:28 2021 +0000
KVM: Use interval tree to do fast hva lookup in memslots
bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=142aa59d700000
final oops: https://syzkaller.appspot.com/x/report.txt?x=162aa59d700000
console output: https://syzkaller.appspot.com/x/log.txt?x=122aa59d700000
It bisects here just because the patch introduces the warning; the issue
is a mmu_notifier_invalidate_range_start with an empty range. The
offending system call
mremap(&(0x7f000000d000/0x2000)=nil, 0xfffffffffffffe74, 0x1000, 0x3,
&(0x7f0000007000/0x1000)=nil)
really means old_len == 0 (it's page-aligned at the beginning of
sys_mremap), and flags includes MREMAP_FIXED so it goes down to
mremap_to and from there to move_page_tables. No function on this path
attempts to special case old_len == 0, the immediate fix would be
diff --git a/mm/mremap.c b/mm/mremap.c
index 002eec83e91e..0e175aef536e 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -486,6 +486,9 @@ unsigned long move_page_tables(struct vm_area_struct
pmd_t *old_pmd, *new_pmd;
pud_t *old_pud, *new_pud;
+ if (!len)
+ return 0;
+
old_end = old_addr + len;
flush_cache_range(vma, old_addr, old_end);
but there are several other ways to fix this elsewhere in the call chain:
- check for old_len == 0 somewhere in mremap_to
- skip the call in __mmu_notifier_invalidate_range_start and
__mmu_notifier_invalidate_range_end, if people agree not to play
whack-a-mole with the callers of mmu_notifier_invalidate_range_*.
- remove the warning in KVM
Thanks,
Paolo
