Re: [syzbot] [mm?] general protection fault in shmem_mapping

[Zi Yan <ziy@nvidia.com>]

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On 14 Feb 2025, at 10:48, syzbot wrote:

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    df5d6180169a Add linux-next specific files for 20250210
> git tree:       linux-next
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=16d343f8580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=2cd616b6e736ca7
> dashboard link: https://syzkaller.appspot.com/bug?extid=012c6245eaea0e23f7f9
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=138972a4580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11737b18580000
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/92495cf20271/disk-df5d6180.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/4b3656161842/vmlinux-df5d6180.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/7fbc4e7a0f5d/bzImage-df5d6180.xz
>
> The issue was bisected to:
>
> commit 19c067c8a0f18b62d4a4235b0b327624c86f2e40
> Author: Zi Yan <ziy@nvidia.com>
> Date:   Wed Feb 5 03:14:13 2025 +0000
>
>     mm/huge_memory: add buddy allocator like folio_split()
>
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=102f9bdf980000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=122f9bdf980000
> console output: https://syzkaller.appspot.com/x/log.txt?x=142f9bdf980000
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+012c6245eaea0e23f7f9@syzkaller.appspotmail.com
> Fixes: 19c067c8a0f1 ("mm/huge_memory: add buddy allocator like folio_split()")
>
> Oops: general protection fault, probably for non-canonical address 0xdffffc0000000024: 0000 [#1] PREEMPT SMP KASAN PTI
> KASAN: null-ptr-deref in range [0x0000000000000120-0x0000000000000127]
> CPU: 1 UID: 0 PID: 5949 Comm: syz-executor163 Not tainted 6.14.0-rc2-next-20250210-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 12/27/2024
> RIP: 0010:shmem_mapping+0x25/0x50 mm/shmem.c:281
> Code: 90 90 90 90 90 f3 0f 1e fa 53 48 89 fb e8 c3 3a bd ff 48 81 c3 20 01 00 00 48 89 d8 48 c1 e8 03 48 b9 00 00 00 00 00 fc ff df <80> 3c 08 00 74 08 48 89 df e8 dd 35 24 00 48 c7 c0 80 6c 14 8c 48
> RSP: 0018:ffffc90004036dd8 EFLAGS: 00010006
> RAX: 0000000000000024 RBX: 0000000000000120 RCX: dffffc0000000000
> RDX: 0000000000000000 RSI: 0000000000000600 RDI: 0000000000000000
> RBP: 0000000000000600 R08: ffffffff822bcf18 R09: 1ffffd4000331008
> R10: dffffc0000000000 R11: fffff94000331009 R12: ffffea0001988018
> R13: ffffea0001988080 R14: ffffea0001988018 R15: ffffea0001988000
> FS:  00007f5e045316c0(0000) GS:ffff8880b8700000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f5e0458dae0 CR3: 0000000023c78000 CR4: 00000000003526f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  __split_unmapped_folio+0x238a/0x3e50 mm/huge_memory.c:3587
>  __folio_split+0x1267/0x16d0 mm/huge_memory.c:3871
>  try_folio_split include/linux/huge_mm.h:374 [inline]
>  truncate_inode_partial_folio+0x55d/0xdc0 mm/truncate.c:230
>  shmem_undo_range+0xab7/0x1820 mm/shmem.c:1144
>  shmem_truncate_range mm/shmem.c:1224 [inline]
>  shmem_fallocate+0x431/0xf20 mm/shmem.c:3674
>  vfs_fallocate+0x623/0x7a0 fs/open.c:338
>  madvise_remove mm/madvise.c:1034 [inline]
>  madvise_vma_behavior mm/madvise.c:1269 [inline]
>  madvise_walk_vmas mm/madvise.c:1511 [inline]
>  madvise_do_behavior+0x1f29/0x3c00 mm/madvise.c:1646
>  do_madvise mm/madvise.c:1744 [inline]
>  __do_sys_madvise mm/madvise.c:1752 [inline]
>  __se_sys_madvise mm/madvise.c:1750 [inline]
>  __x64_sys_madvise+0x11b/0x140 mm/madvise.c:1750
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f5e04576519
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 51 18 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007f5e04531218 EFLAGS: 00000246 ORIG_RAX: 000000000000001c
> RAX: ffffffffffffffda RBX: 00007f5e04600308 RCX: 00007f5e04576519
> RDX: 0000000000000009 RSI: 0000000000600003 RDI: 0000400000000000
> RBP: 00007f5e04600300 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007f5e045cd074
> R13: 0000400000000000 R14: 0000400000000140 R15: 0000400000000040
>  </TASK>
> Modules linked in:
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:shmem_mapping+0x25/0x50 mm/shmem.c:281
> Code: 90 90 90 90 90 f3 0f 1e fa 53 48 89 fb e8 c3 3a bd ff 48 81 c3 20 01 00 00 48 89 d8 48 c1 e8 03 48 b9 00 00 00 00 00 fc ff df <80> 3c 08 00 74 08 48 89 df e8 dd 35 24 00 48 c7 c0 80 6c 14 8c 48
> RSP: 0018:ffffc90004036dd8 EFLAGS: 00010006
> RAX: 0000000000000024 RBX: 0000000000000120 RCX: dffffc0000000000
> RDX: 0000000000000000 RSI: 0000000000000600 RDI: 0000000000000000
> RBP: 0000000000000600 R08: ffffffff822bcf18 R09: 1ffffd4000331008
> R10: dffffc0000000000 R11: fffff94000331009 R12: ffffea0001988018
> R13: ffffea0001988080 R14: ffffea0001988018 R15: ffffea0001988000
> FS:  00007f5e045316c0(0000) GS:ffff8880b8700000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f5e0458dae0 CR3: 0000000023c78000 CR4: 00000000003526f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> ----------------
> Code disassembly (best guess):
>    0:	90                   	nop
>    1:	90                   	nop
>    2:	90                   	nop
>    3:	90                   	nop
>    4:	90                   	nop
>    5:	f3 0f 1e fa          	endbr64
>    9:	53                   	push   %rbx
>    a:	48 89 fb             	mov    %rdi,%rbx
>    d:	e8 c3 3a bd ff       	call   0xffbd3ad5
>   12:	48 81 c3 20 01 00 00 	add    $0x120,%rbx
>   19:	48 89 d8             	mov    %rbx,%rax
>   1c:	48 c1 e8 03          	shr    $0x3,%rax
>   20:	48 b9 00 00 00 00 00 	movabs $0xdffffc0000000000,%rcx
>   27:	fc ff df
> * 2a:	80 3c 08 00          	cmpb   $0x0,(%rax,%rcx,1) <-- trapping instruction
>   2e:	74 08                	je     0x38
>   30:	48 89 df             	mov    %rbx,%rdi
>   33:	e8 dd 35 24 00       	call   0x243615
>   38:	48 c7 c0 80 6c 14 8c 	mov    $0xffffffff8c146c80,%rax
>   3f:	48                   	rex.W

OK, I figured out how to trigger syzkaller to do the test. The attached
patch includes both the fix and the __split_folio_to_order() change
discussed at https://lore.kernel.org/linux-mm/d989ef66-4a4b-4679-b2e9-f05916ca3672@redhat.com/. I will paste the patch to my original patch
email once syzkaller confirms it fixes the issue.


#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git df5d6180169a


Best Regards,
Yan, Zi

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From a6bd83dfbb1143f1614ede4817cccb1e8cc6290d Mon Sep 17 00:00:00 2001
From: Zi Yan <ziy@nvidia.com>
Date: Fri, 14 Feb 2025 16:18:24 -0500
Subject: [PATCH] mm/huge_memory: do not drop the original folio during
 truncate.

The caller expects to handle the original folio itself.

also make __split_unmapped_folio() never fail, per discussion with David
Hildenbrand.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 mm/huge_memory.c | 22 ++++++----------------
 1 file changed, 6 insertions(+), 16 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 2eda2a9ec8fc..87cb62c81bf3 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3292,16 +3292,12 @@ bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
  * It splits @folio into @new_order folios and copies the @folio metadata to
  * all the resulting folios.
  */
-static int __split_folio_to_order(struct folio *folio, int new_order)
+static void __split_folio_to_order(struct folio *folio, int new_order)
 {
-	int curr_order = folio_order(folio);
 	long nr_pages = folio_nr_pages(folio);
 	long new_nr_pages = 1 << new_order;
 	long index;
 
-	if (curr_order <= new_order)
-		return -EINVAL;
-
 	/*
 	 * Skip the first new_nr_pages, since the new folio from them have all
 	 * the flags from the original folio.
@@ -3396,8 +3392,6 @@ static int __split_folio_to_order(struct folio *folio, int new_order)
 
 	if (!new_order)
 		ClearPageCompound(&folio->page);
-
-	return 0;
 }
 
 /*
@@ -3491,7 +3485,6 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
 		int old_order = folio_order(folio);
 		struct folio *release;
 		struct folio *end_folio = folio_next(folio);
-		int status;
 
 		/* order-1 anonymous folio is not supported */
 		if (folio_test_anon(folio) && split_order == 1)
@@ -3524,12 +3517,7 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
 		split_page_owner(&folio->page, old_order, split_order);
 		pgalloc_tag_split(folio, old_order, split_order);
 
-		status = __split_folio_to_order(folio, split_order);
-
-		if (status < 0) {
-			stop_split = true;
-			ret = -EINVAL;
-		}
+		__split_folio_to_order(folio, split_order);
 
 after_split:
 		/*
@@ -3567,8 +3555,10 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
 				     folio_test_swapcache(origin_folio)) ?
 					     folio_nr_pages(release) : 0));
 
-			if (release != origin_folio)
-				lru_add_page_tail(origin_folio, &release->page,
+			if (release == origin_folio)
+				continue;
+			
+			lru_add_page_tail(origin_folio, &release->page,
 						lruvec, list);
 
 			/* Some pages can be beyond EOF: drop them from page cache */
-- 
2.47.2