On Wed, 2025-08-27 at 20:17 +0200, Max Kellermann wrote:
> The function ceph_process_folio_batch() sets folio_batch entries to
> NULL, which is an illegal state. Before folio_batch_release() crashes
> due to this API violation, the function
> ceph_shift_unused_folios_left() is supposed to remove those NULLs from
> the array.
>
> However, since commit ce80b76dd327 ("ceph: introduce
> ceph_process_folio_batch() method"), this shifting doesn't happen
> anymore because the "for" loop got moved to
> ceph_process_folio_batch(), and now the `i` variable that remains in
> ceph_writepages_start() doesn't get incremented anymore, making the
> shifting effectively unreachable much of the time.
>
> Later, commit 1551ec61dc55 ("ceph: introduce ceph_submit_write()
> method") added more preconditions for doing the shift, replacing the
> `i` check (with something that is still just as broken):
>
> - if ceph_process_folio_batch() fails, shifting never happens
>
> - if ceph_move_dirty_page_in_page_array() was never called (because
> ceph_process_folio_batch() has returned early for some of various
> reasons), shifting never happens
>
> - if `processed_in_fbatch` is zero (because ceph_process_folio_batch()
> has returned early for some of the reasons mentioned above or
> because ceph_move_dirty_page_in_page_array() has failed), shifting
> never happens
>
> Since those two commits, any problem in ceph_process_folio_batch()
> could crash the kernel, e.g. this way:
>
> BUG: kernel NULL pointer dereference, address: 0000000000000034
> #PF: supervisor write access in kernel mode
> #PF: error_code(0x0002) - not-present page
> PGD 0 P4D 0
> Oops: Oops: 0002 [#1] SMP NOPTI
> CPU: 172 UID: 0 PID: 2342707 Comm: kworker/u778:8 Not tainted 6.15.10-cm4all1-es #714 NONE
> Hardware name: Dell Inc. PowerEdge R7615/0G9DHV, BIOS 1.6.10 12/08/2023
> Workqueue: writeback wb_workfn (flush-ceph-1)
> RIP: 0010:folios_put_refs+0x85/0x140
> Code: 83 c5 01 39 e8 7e 76 48 63 c5 49 8b 5c c4 08 b8 01 00 00 00 4d 85 ed 74 05 41 8b 44 ad 00 48 8b 15 b0 >
> RSP: 0018:ffffb880af8db778 EFLAGS: 00010207
> RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000000000003
> RDX: ffffe377cc3b0000 RSI: 0000000000000000 RDI: ffffb880af8db8c0
> RBP: 0000000000000000 R08: 000000000000007d R09: 000000000102b86f
> R10: 0000000000000001 R11: 00000000000000ac R12: ffffb880af8db8c0
> R13: 0000000000000000 R14: 0000000000000000 R15: ffff9bd262c97000
> FS: 0000000000000000(0000) GS:ffff9c8efc303000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000034 CR3: 0000000160958004 CR4: 0000000000770ef0
> PKRU: 55555554
> Call Trace:
> <TASK>
> ceph_writepages_start+0xeb9/0x1410
>
> The crash can be reproduced easily by changing the
> ceph_check_page_before_write() return value to `-E2BIG`.
>
I cannot reproduce the crash/issue. If ceph_check_page_before_write() returns
`-E2BIG`, then nothing happens. There is no crush and no write operations could
be processed by file system driver anymore. So, it doesn't look like recipe to
reproduce the issue. I cannot confirm that the patch fixes the issue without
clear way to reproduce the issue.
Could you please provide more clear explanation of the issue reproduction path?
Thanks,
Slava.
> (Interestingly, the crash happens only if `huge_zero_folio` has
> already been allocated; without `huge_zero_folio`,
> is_huge_zero_folio(NULL) returns true and folios_put_refs() skips NULL
> entries instead of dereferencing them. That makes reproducing the bug
> somewhat unreliable. See
> https://lore.kernel.org/20250826231626.218675-1-max.kellermann@xxxxxxxxx
> for a discussion of this detail.)
>
> My suggestion is to move the ceph_shift_unused_folios_left() to right
> after ceph_process_folio_batch() to ensure it always gets called to
> fix up the illegal folio_batch state.
>
> Fixes: ce80b76dd327 ("ceph: introduce ceph_process_folio_batch() method")
> Link: https://lore.kernel.org/ceph-devel/aK4v548CId5GIKG1@xxxxxxxxxxxxxx/
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Max Kellermann <max.kellermann@xxxxxxxxx>
> ---
> fs/ceph/addr.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> index 8b202d789e93..8bc66b45dade 100644
> --- a/fs/ceph/addr.c
> +++ b/fs/ceph/addr.c
> @@ -1687,6 +1687,7 @@ static int ceph_writepages_start(struct address_space *mapping,
>
> process_folio_batch:
> rc = ceph_process_folio_batch(mapping, wbc, &ceph_wbc);
> + ceph_shift_unused_folios_left(&ceph_wbc.fbatch);
> if (rc)
> goto release_folios;
>
> @@ -1695,8 +1696,6 @@ static int ceph_writepages_start(struct address_space *mapping,
> goto release_folios;
>
> if (ceph_wbc.processed_in_fbatch) {
> - ceph_shift_unused_folios_left(&ceph_wbc.fbatch);
> -
> if (folio_batch_count(&ceph_wbc.fbatch) == 0 &&
> ceph_wbc.locked_pages < ceph_wbc.max_pages) {
> doutc(cl, "reached end fbatch, trying for more\n");
