[syzbot] [fs?] [mm?] kernel BUG in __filemap_add_folio

[syzbot] [fs?] [mm?] kernel BUG in __filemap_add_folio

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    ac71fabf1567 gcc-15: work around sequence-point warning
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1269b204580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=68d9f79fc685cd4
dashboard link: https://syzkaller.appspot.com/bug?extid=4d3cc33ef7a77041efa6
compiler:       Debian clang version 15.0.6, Debian LLD 15.0.6
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=10b2cc70580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14a91ccc580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/c03ec6447343/disk-ac71fabf.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/e02e7fb54511/vmlinux-ac71fabf.xz
kernel image: https://storage.googleapis.com/syzbot-assets/7d53dcc94699/bzImage-ac71fabf.xz

The issue was bisected to:

commit 47dd67532303803a87f43195e088b3b4bcf0454d
Author: Luis Chamberlain <mcgrof@kernel.org>
Date:   Fri Feb 21 22:38:22 2025 +0000

    block/bdev: lift block size restrictions to 64k

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=11d62c70580000
final oops:     https://syzkaller.appspot.com/x/report.txt?x=13d62c70580000
console output: https://syzkaller.appspot.com/x/log.txt?x=15d62c70580000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+4d3cc33ef7a77041efa6@syzkaller.appspotmail.com
Fixes: 47dd67532303 ("block/bdev: lift block size restrictions to 64k")

 __handle_mm_fault mm/memory.c:6140 [inline]
 handle_mm_fault+0x1129/0x1bf0 mm/memory.c:6309
 do_user_addr_fault arch/x86/mm/fault.c:1337 [inline]
 handle_page_fault arch/x86/mm/fault.c:1480 [inline]
 exc_page_fault+0x45b/0x920 arch/x86/mm/fault.c:1538
 asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:623
------------[ cut here ]------------
kernel BUG at mm/filemap.c:868!
Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
CPU: 1 UID: 0 PID: 5909 Comm: syz-executor413 Not tainted 6.15.0-rc2-syzkaller-00493-gac71fabf1567 #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
RIP: 0010:__filemap_add_folio+0x1554/0x16c0 mm/filemap.c:867
Code: e9 c5 ff 4c 89 e7 48 c7 c6 80 0e 54 8c e8 44 43 12 00 90 0f 0b e8 cc e9 c5 ff 4c 89 e7 48 c7 c6 00 05 54 8c e8 2d 43 12 00 90 <0f> 0b e8 b5 e9 c5 ff 4c 89 e7 48 c7 c6 80 0e 54 8c e8 16 43 12 00
RSP: 0018:ffffc90004087300 EFLAGS: 00010246
RAX: ba56fbab94ec7e00 RBX: 0000000000000004 RCX: ffffffff93686020
RDX: dffffc0000000000 RSI: ffffffff8e6497f7 RDI: 0000000000000001
RBP: ffffc900040874b0 R08: ffffffff905fe577 R09: 1ffffffff20bfcae
R10: dffffc0000000000 R11: fffffbfff20bfcaf R12: ffffea0001ededc0
R13: ffffc90004087400 R14: 0000000000000000 R15: dffffc0000000000
FS:  0000555577940380(0000) GS:ffff88812509a000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fb2a265a1f0 CR3: 000000007eb78000 CR4: 0000000000350ef0
Call Trace:
 <TASK>
 filemap_add_folio+0x157/0x380 mm/filemap.c:969
 page_cache_ra_unbounded+0x40c/0x820 mm/readahead.c:275
 do_sync_mmap_readahead+0x3e6/0x6c0 mm/filemap.c:-1
 filemap_fault+0x763/0x13d0 mm/filemap.c:3403
 __do_fault+0x137/0x390 mm/memory.c:5098
 do_shared_fault mm/memory.c:5582 [inline]
 do_fault mm/memory.c:5656 [inline]
 do_pte_missing mm/memory.c:4160 [inline]
 handle_pte_fault+0xfcc/0x61c0 mm/memory.c:5997
 __handle_mm_fault mm/memory.c:6140 [inline]
 handle_mm_fault+0x1129/0x1bf0 mm/memory.c:6309
 do_user_addr_fault arch/x86/mm/fault.c:1337 [inline]
 handle_page_fault arch/x86/mm/fault.c:1480 [inline]
 exc_page_fault+0x45b/0x920 arch/x86/mm/fault.c:1538
 asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:623
RIP: 0033:0x7fb2a25adba4
Code: 2d 41 b8 13 00 00 00 bf 09 00 00 00 bb 03 00 00 00 e8 b0 14 03 00 ba 71 12 08 40 bf 10 00 00 00 48 b8 00 01 00 00 00 20 00 00 <48> c7 00 00 00 01 00 48 8b 35 1e 95 0a 00 48 89 c1 31 c0 e8 84 14
RSP: 002b:00007ffd9b4d3c20 EFLAGS: 00010217
RAX: 0000200000000100 RBX: 0000000000000003 RCX: 00007fb2a25df059
RDX: 0000000040081271 RSI: 0000000000b36000 RDI: 0000000000000010
RBP: 0000000000000000 R08: 0000000000000004 R09: 0000000000000000
R10: 0000000000000013 R11: 0000000000000216 R12: 00007ffd9b4d3c4c
R13: 00007ffd9b4d3c60 R14: 00007ffd9b4d3ca0 R15: 0000000000000008
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:__filemap_add_folio+0x1554/0x16c0 mm/filemap.c:867
Code: e9 c5 ff 4c 89 e7 48 c7 c6 80 0e 54 8c e8 44 43 12 00 90 0f 0b e8 cc e9 c5 ff 4c 89 e7 48 c7 c6 00 05 54 8c e8 2d 43 12 00 90 <0f> 0b e8 b5 e9 c5 ff 4c 89 e7 48 c7 c6 80 0e 54 8c e8 16 43 12 00
RSP: 0018:ffffc90004087300 EFLAGS: 00010246
RAX: ba56fbab94ec7e00 RBX: 0000000000000004 RCX: ffffffff93686020
RDX: dffffc0000000000 RSI: ffffffff8e6497f7 RDI: 0000000000000001
RBP: ffffc900040874b0 R08: ffffffff905fe577 R09: 1ffffffff20bfcae
R10: dffffc0000000000 R11: fffffbfff20bfcaf R12: ffffea0001ededc0
R13: ffffc90004087400 R14: 0000000000000000 R15: dffffc0000000000
FS:  0000555577940380(0000) GS:ffff888124f9a000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fb2a265a1f0 CR3: 000000007eb78000 CR4: 0000000000350ef0


---
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If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
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Re: [syzbot] [fs?] [mm?] kernel BUG in __filemap_add_folio

[shaurya]

[-- Attachment #1: Type: text/plain, Size: 83 bytes --]

#syz test:
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

[-- Attachment #2: 0001-mm-readahead-fix-race-between-page_cache_ra_order-an.patch --]
[-- Type: text/x-patch, Size: 3056 bytes --]

From ec7ea9a1f03f36672cf5acb23761cfef6b948f21 Mon Sep 17 00:00:00 2001
From: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>
Date: Sun, 30 Nov 2025 20:27:25 +0530
Subject: [PATCH] mm/readahead: fix race between page_cache_ra_order and
 set_blocksize

page_cache_ra_order() reads mapping_min_folio_order() before acquiring
the invalidate_lock, creating a time-of-check-time-of-use (TOCTOU) race
with set_blocksize() which can change the mapping's min_folio_order
while holding the invalidate_lock exclusively.

If set_blocksize() increases the mapping's min_folio_order after
page_cache_ra_order() reads the old value but before it adds folios
to the page cache, the VM_BUG_ON check in __filemap_add_folio() will
trigger:

  VM_BUG_ON_FOLIO(folio_order(folio) < mapping_min_folio_order(mapping),
                  folio);

This can happen because the stale min_order is used to calculate
new_order and constrain the folio order, but filemap_add_folio()
re-reads the (now increased) min_folio_order from the mapping.

Fix this by moving the read of mapping_min_folio_order() and the
new_order calculation to after the invalidate_lock is acquired in
shared mode.

Reported-by: syzbot+4d3cc33ef7a77041efa6@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug\?extid\=4d3cc33ef7a77041efa6
Fixes: 47dd67532303 ("block/bdev: lift block size restrictions to 64k")
Cc: stable@vger.kernel.org
Signed-off-by: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>
---
 mm/readahead.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index 3a4b5d58eeb6..95718f87bd43 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -467,7 +467,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
 	struct address_space *mapping = ractl->mapping;
 	pgoff_t start = readahead_index(ractl);
 	pgoff_t index = start;
-	unsigned int min_order = mapping_min_folio_order(mapping);
+	unsigned int min_order;
 	pgoff_t limit = (i_size_read(mapping->host) - 1) >> PAGE_SHIFT;
 	pgoff_t mark = index + ra->size - ra->async_size;
 	unsigned int nofs;
@@ -483,15 +483,22 @@ void page_cache_ra_order(struct readahead_control *ractl,
 
 	limit = min(limit, index + ra->size - 1);
 
+	/* See comment in page_cache_ra_unbounded() */
+	nofs = memalloc_nofs_save();
+	filemap_invalidate_lock_shared(mapping);
+
+	/*
+	 * Re-read min_order after acquiring the invalidate_lock to avoid a
+	 * race with set_blocksize() which can change the mapping's min_order
+	 * while holding the invalidate_lock exclusively.
+	 */
+	min_order = mapping_min_folio_order(mapping);
 	new_order = min(mapping_max_folio_order(mapping), new_order);
 	new_order = min_t(unsigned int, new_order, ilog2(ra->size));
 	new_order = max(new_order, min_order);
 
 	ra->order = new_order;
 
-	/* See comment in page_cache_ra_unbounded() */
-	nofs = memalloc_nofs_save();
-	filemap_invalidate_lock_shared(mapping);
 	/*
 	 * If the new_order is greater than min_order and index is
 	 * already aligned to new_order, then this will be noop as index
-- 
2.34.1

Re: [syzbot] [fs?] [mm?] kernel BUG in __filemap_add_folio

[syzbot]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
kernel BUG in mpage_readahead

------------[ cut here ]------------
kernel BUG at ./include/linux/pagemap.h:1408!
Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
CPU: 1 UID: 0 PID: 18176 Comm: syz-executor317 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
RIP: 0010:__readahead_folio include/linux/pagemap.h:1408 [inline]
RIP: 0010:readahead_folio include/linux/pagemap.h:1434 [inline]
RIP: 0010:mpage_readahead+0x4ad/0x5a0 fs/mpage.c:367
Code: 5e 41 5f c3 cc cc cc cc e8 b0 19 70 ff 48 89 ef e8 48 06 ad ff e9 54 fe ff ff 4c 8b 6c 24 18 e9 43 ff ff ff e8 94 19 70 ff 90 <0f> 0b e8 8c 19 70 ff 48 c7 c6 00 b6 80 8b 48 89 ef e8 1d 39 ba ff
RSP: 0018:ffffc9000d92f620 EFLAGS: 00010293
RAX: 0000000000000000 RBX: ffffc9000d92fae8 RCX: ffffffff824c7d14
RDX: ffff88802fe68000 RSI: ffffffff824c809c RDI: 0000000000000004
RBP: 0000000000000004 R08: 0000000000000004 R09: 0000000000000004
R10: 0000000000000001 R11: 0000000000000000 R12: dffffc0000000000
R13: fffff52001b25f61 R14: 0000000000000001 R15: 1ffff92001b25f61
FS:  0000555564bcd380(0000) GS:ffff888124f53000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f08297b0130 CR3: 0000000030a16000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 read_pages+0x1c4/0xc70 mm/readahead.c:163
 page_cache_ra_unbounded+0x66a/0xa10 mm/readahead.c:269
 do_page_cache_ra mm/readahead.c:332 [inline]
 page_cache_ra_order+0xc0b/0xf20 mm/readahead.c:542
 do_sync_mmap_readahead mm/filemap.c:3340 [inline]
 filemap_fault+0x1583/0x29a0 mm/filemap.c:3489
 __do_fault+0x10d/0x490 mm/memory.c:5281
 do_shared_fault mm/memory.c:5780 [inline]
 do_fault mm/memory.c:5854 [inline]
 do_pte_missing+0x1a6/0x3ba0 mm/memory.c:4362
 handle_pte_fault mm/memory.c:6195 [inline]
 __handle_mm_fault+0x1556/0x2aa0 mm/memory.c:6336
 handle_mm_fault+0x589/0xd10 mm/memory.c:6505
 do_user_addr_fault+0x60c/0x1370 arch/x86/mm/fault.c:1336
 handle_page_fault arch/x86/mm/fault.c:1476 [inline]
 exc_page_fault+0x64/0xc0 arch/x86/mm/fault.c:1532
 asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
RIP: 0033:0x7f082970586d
Code: 03 00 b9 03 10 12 00 45 31 c0 48 ba 80 00 00 00 00 20 00 00 48 b8 2f 64 65 76 2f 6e 75 6c 48 c7 c6 9c ff ff ff bf 01 01 00 00 <48> 89 02 48 b8 88 00 00 00 00 20 00 00 c7 00 6c 62 30 00 31 c0 e8
RSP: 002b:00007ffff735c180 EFLAGS: 00010246
RAX: 6c756e2f7665642f RBX: 0000000000000000 RCX: 0000000000121003
RDX: 0000200000000080 RSI: ffffffffffffff9c RDI: 0000000000000101
RBP: 00000000000f4240 R08: 0000000000000000 R09: 0000000000002000
R10: 0000000000000013 R11: 0000000000000206 R12: 0000000000079470
R13: 00007ffff735c19c R14: 00007ffff735c1b0 R15: 00007ffff735c1a0
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:__readahead_folio include/linux/pagemap.h:1408 [inline]
RIP: 0010:readahead_folio include/linux/pagemap.h:1434 [inline]
RIP: 0010:mpage_readahead+0x4ad/0x5a0 fs/mpage.c:367
Code: 5e 41 5f c3 cc cc cc cc e8 b0 19 70 ff 48 89 ef e8 48 06 ad ff e9 54 fe ff ff 4c 8b 6c 24 18 e9 43 ff ff ff e8 94 19 70 ff 90 <0f> 0b e8 8c 19 70 ff 48 c7 c6 00 b6 80 8b 48 89 ef e8 1d 39 ba ff
RSP: 0018:ffffc9000d92f620 EFLAGS: 00010293
RAX: 0000000000000000 RBX: ffffc9000d92fae8 RCX: ffffffff824c7d14
RDX: ffff88802fe68000 RSI: ffffffff824c809c RDI: 0000000000000004
RBP: 0000000000000004 R08: 0000000000000004 R09: 0000000000000004
R10: 0000000000000001 R11: 0000000000000000 R12: dffffc0000000000
R13: fffff52001b25f61 R14: 0000000000000001 R15: 1ffff92001b25f61
FS:  0000555564bcd380(0000) GS:ffff888124f53000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f08297b0130 CR3: 0000000030a16000 CR4: 00000000003526f0


Tested on:

commit:         6bda50f4 Merge tag 'mips-fixes_6.18_2' of git://git.ke..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11857514580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=f4a1ecf59be91960
dashboard link: https://syzkaller.appspot.com/bug?extid=4d3cc33ef7a77041efa6
compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
patch:          https://syzkaller.appspot.com/x/patch.diff?x=10dc7cb4580000