linux-ext4 - [PATCH] ext4: fix infinite loop when replaying fast

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Date: Fri, 10 May 2024 12:52:52 +0100
From: "Luis Henriques (SUSE)" <luis.henriques@...ux.dev>
To: Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger@...ger.ca>,
	Zhang Yi <yi.zhang@...weicloud.com>,
	Harshad Shirwadkar <harshadshirwadkar@...il.com>
Cc: linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"Luis Henriques (SUSE)" <luis.henriques@...ux.dev>
Subject: [PATCH] ext4: fix infinite loop when replaying fast_commit

When doing fast_commit replay an infinite loop may occur due to an
uninitialized extent_status struct.  ext4_ext_determine_insert_hole() does
not detect the replay and calls ext4_es_find_extent_range(), which will
return immediately without initializing the 'es' variable.

Because 'es' contains garbage, an integer overflow may happen causing an
infinite loop in this function, easily reproducible using fstest generic/039.

This commit fixes this issue by detecting the replay in function
ext4_ext_determine_insert_hole().  It also adds initialization code to the
error path in function ext4_es_find_extent_range().

Thanks to Zhang Yi, for figuring out the real problem!

Fixes: 8016e29f4362 ("ext4: fast commit recovery path")
Signed-off-by: Luis Henriques (SUSE) <luis.henriques@...ux.dev>
---
Hi!

Two comments:
1) The change in ext4_ext_map_blocks() could probably use the min_not_zero
   macro instead.  I decided not to do so simply because I wasn't sure if
   that would be safe, but I'm fine changing that if you think it is.

2) I thought about returning 'EXT_MAX_BLOCKS' instead of '0' in
   ext4_lblk_t ext4_ext_determine_insert_hole(), which would then avoid
   the extra change to ext4_ext_map_blocks().  '0' sounds like the right
   value to return, but I'm also OK using 'EXT_MAX_BLOCKS' instead.

And again thanks to Zhang Yi for pointing me the *real* problem!

 fs/ext4/extents.c        | 6 +++++-
 fs/ext4/extents_status.c | 5 ++++-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index e57054bdc5fd..b5bfcb6c18a0 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -4052,6 +4052,9 @@ static ext4_lblk_t ext4_ext_determine_insert_hole(struct inode *inode,
 	ext4_lblk_t hole_start, len;
 	struct extent_status es;

+	if (EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY)
+		return 0;
+
 	hole_start = lblk;
 	len = ext4_ext_find_hole(inode, path, &hole_start);
 again:
@@ -4226,7 +4229,8 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 		len = ext4_ext_determine_insert_hole(inode, path, map->m_lblk);

 		map->m_pblk = 0;
-		map->m_len = min_t(unsigned int, map->m_len, len);
+		if (len > 0)
+			map->m_len = min_t(unsigned int, map->m_len, len);
 		goto out;
 	}

diff --git a/fs/ext4/extents_status.c b/fs/ext4/extents_status.c
index 4a00e2f019d9..acb9616ca119 100644
--- a/fs/ext4/extents_status.c
+++ b/fs/ext4/extents_status.c
@@ -310,8 +310,11 @@ void ext4_es_find_extent_range(struct inode *inode,
 			       ext4_lblk_t lblk, ext4_lblk_t end,
 			       struct extent_status *es)
 {
-	if (EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY)
+	if (EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY) {
+		/* Initialize extent to zero */
+		es->es_lblk = es->es_len = es->es_pblk = 0;
 		return;
+	}

 	trace_ext4_es_find_extent_range_enter(inode, lblk);