On Mar 30 19:53, Ivan Orlov wrote: > Syzkaller reported the following issue: > > kernel BUG at mm/khugepaged.c:1823! > invalid opcode: 0000 [#1] PREEMPT SMP KASAN > CPU: 1 PID: 5097 Comm: syz-executor220 Not tainted 6.2.0-syzkaller-13154-g857f1268a591 #0 > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/16/2023 > RIP: 0010:collapse_file mm/khugepaged.c:1823 [inline] > RIP: 0010:hpage_collapse_scan_file+0x67c8/0x7580 mm/khugepaged.c:2233 > Code: 00 00 89 de e8 c9 66 a3 ff 31 ff 89 de e8 c0 66 a3 ff 45 84 f6 0f 85 28 0d 00 00 e8 22 64 a3 ff e9 dc f7 ff ff e8 18 64 a3 ff <0f> 0b f3 0f 1e fa e8 0d 64 a3 ff e9 93 f6 ff ff f3 0f 1e fa 4c 89 > RSP: 0018:ffffc90003dff4e0 EFLAGS: 00010093 > RAX: ffffffff81e95988 RBX: 00000000000001c1 RCX: ffff8880205b3a80 > RDX: 0000000000000000 RSI: 00000000000001c0 RDI: 00000000000001c1 > RBP: ffffc90003dff830 R08: ffffffff81e90e67 R09: fffffbfff1a433c3 > R10: 0000000000000000 R11: dffffc0000000001 R12: 0000000000000000 > R13: ffffc90003dff6c0 R14: 00000000000001c0 R15: 0000000000000000 > FS: 00007fdbae5ee700(0000) GS:ffff8880b9900000(0000) knlGS:0000000000000000 > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > CR2: 00007fdbae6901e0 CR3: 000000007b2dd000 CR4: 00000000003506e0 > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 > Call Trace: > <TASK> > madvise_collapse+0x721/0xf50 mm/khugepaged.c:2693 > madvise_vma_behavior mm/madvise.c:1086 [inline] > madvise_walk_vmas mm/madvise.c:1260 [inline] > do_madvise+0x9e5/0x4680 mm/madvise.c:1439 > __do_sys_madvise mm/madvise.c:1452 [inline] > __se_sys_madvise mm/madvise.c:1450 [inline] > __x64_sys_madvise+0xa5/0xb0 mm/madvise.c:1450 > do_syscall_x64 arch/x86/entry/common.c:50 [inline] > do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80 > entry_SYSCALL_64_after_hwframe+0x63/0xcd > Thanks, Ivan. In the process of reviewing this, I starting thinking if the !shmem case was also susceptible to a similar race, and I *think* it might be. Unfortunately, my time has ran out, and I haven't been able to validate ; I'm less familiar with the file-side of things. The underlying problem is race with truncation/hole-punch under OOM condition. The nice do-while loop near the top of collapse_file() attempts to avoid this scenario by making sure enough slots are available. However, when we drop xarray lock, we open ourselves up to concurrent removal + slot deletion. Those slots then need to be allocated again -- which under OOM condition is failable. The syzbot reproducer picks on shmem, but I think this can occur for file as well. If we find a hole, we unlock the xarray and call page_cache_sync_readahead(), which if it succeeds, IIUC, will have allocated a new slot in our mapping pointing to the new page. We *then* locks the page. Only after the page is locked are we protected from concurrent removal (Note: this is what provides us protection in many of the xas_store() cases ; we've held the slot's contained page-lock since verifying the slot exists, protecting us from removal / reallocation races). Maybe I'm just low on caffeine at the end of the day, and am missing something, but if I had more time, I'd be looking into the file-side some more to verify. Apologies that hasn't occurred to me until now ; I was looking at one of your comments and double-checked why I *thought* we were safe. Anyways, irrespective of that looming issues, some more notes to follow: > The 'xas_store' call during page cache scanning can potentially > translate 'xas' into the error state (with the reproducer provided > by the syzkaller the error code is -ENOMEM). However, there are no > further checks after the 'xas_store', and the next call of 'xas_next' > at the start of the scanning cycle doesn't increase the xa_index, > and the issue occurs. > > This patch will add the xarray state error checking after the > 'xas_store' and the corresponding result error code. It will > also add xarray state error checking via WARN_ON_ONCE macros, > to be sure that ENOMEM or other possible errors don't occur > at the places they shouldn't. Thanks for the additions here. I think it's worthwhile providing even more details about the specifics of the race we are fixing and/or guarding against to help ppl understand how that -ENOMEM comes about if the do-while loop has "Ensured" we have slots available (additionally, I think that comment can be augmented). > Tested via syzbot. > > Reported-by: syzbot+9578faa5475acb35fa50@xxxxxxxxxxxxxxxxxxxxxxxxx > Link: https://syzkaller.appspot.com/bug?id=7d6bb3760e026ece7524500fe44fb024a0e959fc > Signed-off-by: Ivan Orlov <ivan.orlov0322@xxxxxxxxx> > --- > V1 -> V2: Add WARN_ON_ONCE error checking and comments > > mm/khugepaged.c | 20 ++++++++++++++++++++ > 1 file changed, 20 insertions(+) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 92e6f56a932d..8b6580b13339 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -55,6 +55,7 @@ enum scan_result { > SCAN_CGROUP_CHARGE_FAIL, > SCAN_TRUNCATED, > SCAN_PAGE_HAS_PRIVATE, > + SCAN_STORE_FAILED, > }; I'm still reluctant to add a new error code for this as this seems like quite a rare race that requires OOM to trigger. I'd be happier just reusing SCAN_FAIL, or, something we might get some millage out of later: SCAN_OOM. Also, a reminder to update include/trace/events/huge_memory.h, if you go that route. > > #define CREATE_TRACE_POINTS > @@ -1840,6 +1841,15 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr, > goto xa_locked; > } > xas_store(&xas, hpage); > + if (xas_error(&xas)) { > + /* revert shmem_charge performed > + * in the previous condition > + */ Nit: Here, and following, I think standard convention for multiline comment is to have an empty first and last line, eg: + /* + * revert shmem_charge performed + * in the previous condition + */ Though, checkpatch.pl --strict didn't seem to care. > + mapping->nrpages--; > + shmem_uncharge(mapping->host, 1); > + result = SCAN_STORE_FAILED; > + goto xa_locked; > + } > nr_none++; > continue; > } > @@ -1992,6 +2002,11 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr, > > /* Finally, replace with the new page. */ > xas_store(&xas, hpage); > + /* We can't get an ENOMEM here (because the allocation happened before) > + * but let's check for errors (XArray implementation can be > + * changed in the future) > + */ > + WARN_ON_ONCE(xas_error(&xas)); Nit: it's not just that allocation happened before -- need some guarantee we've been protected from concurrent removal. This is what made me look at the file side. > continue; > out_unlock: > unlock_page(page); > @@ -2029,6 +2044,11 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr, > /* Join all the small entries into a single multi-index entry */ > xas_set_order(&xas, start, HPAGE_PMD_ORDER); > xas_store(&xas, hpage); > + /* Here we can't get an ENOMEM (because entries were > + * previously allocated) But let's check for errors > + * (XArray implementation can be changed in the future) > + */ > + WARN_ON_ONCE(xas_error(&xas)); Ditto. Apologies I won't be around to see this change through -- I'm just out of time, and will be shutting my computer down tomorrow for 3 months. Sorry for the poor timing, for raising issues, then disappearing. Hopefully I'm wrong and the file-side isn't a concern. Best, Zach > xa_locked: > xas_unlock_irq(&xas); > xa_unlocked: > -- > 2.34.1 >
