linux-kernel - [PATCH] afs: Fix endless loop in directory parsing

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Message-ID: <786185.1708694102@warthog.procyon.org.uk>
Date: Fri, 23 Feb 2024 13:15:02 +0000
From: David Howells <dhowells@...hat.com>
To: Marc Dionne <marc.dionne@...istor.com>
cc: dhowells@...hat.com, Markus Suvanto <markus.suvanto@...il.com>,
    Christian Brauner <brauner@...nel.org>,
    linux-afs@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
    linux-kernel@...r.kernel.org
Subject: [PATCH] afs: Fix endless loop in directory parsing

    
If a directory has a block with only ".__afsXXXX" files in it (from
uncompleted silly-rename), these .__afsXXXX files are skipped but without
advancing the file position in the dir_context.  This leads to
afs_dir_iterate() repeating the block again and again.

Fix this by making the code that skips the .__afsXXXX file also manually
advance the file position.

The symptoms are a soft lookup:

        watchdog: BUG: soft lockup - CPU#3 stuck for 52s! [check:5737]
        ...
        RIP: 0010:afs_dir_iterate_block+0x39/0x1fd
        ...
         ? watchdog_timer_fn+0x1a6/0x213
        ...
         ? asm_sysvec_apic_timer_interrupt+0x16/0x20
         ? afs_dir_iterate_block+0x39/0x1fd
         afs_dir_iterate+0x10a/0x148
         afs_readdir+0x30/0x4a
         iterate_dir+0x93/0xd3
         __do_sys_getdents64+0x6b/0xd4

This is almost certainly the actual fix for:

        https://bugzilla.kernel.org/show_bug.cgi?id=218496

Fixes: 57e9d49c5452 ("afs: Hide silly-rename files from userspace")
Signed-off-by: David Howells <dhowells@...hat.com>
cc: Marc Dionne <marc.dionne@...istor.com>
cc: Markus Suvanto <markus.suvanto@...il.com>
cc: linux-afs@...ts.infradead.org
---
 fs/afs/dir.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/afs/dir.c b/fs/afs/dir.c
index b5b8de521f99..8a67fc427e74 100644
--- a/fs/afs/dir.c
+++ b/fs/afs/dir.c
@@ -479,8 +479,10 @@ static int afs_dir_iterate_block(struct afs_vnode *dvnode,
 		    dire->u.name[0] == '.' &&
 		    ctx->actor != afs_lookup_filldir &&
 		    ctx->actor != afs_lookup_one_filldir &&
-		    memcmp(dire->u.name, ".__afs", 6) == 0)
+		    memcmp(dire->u.name, ".__afs", 6) == 0) {
+			ctx->pos = blkoff + next * sizeof(union afs_xdr_dirent);
 			continue;
+		}
 
 		/* found the next entry */
 		if (!dir_emit(ctx, dire->u.name, nlen,