[PATCH AUTOSEL 6.12 51/93] ext4: prevent stale extent cache entries caused by concurrent get es_cache

[Sasha Levin <sashal@kernel.org>]

From: Zhang Yi <yi.zhang@huawei.com>

[ Upstream commit f22a0ef2231a7d8374bb021eb86404d0e9de5a02 ]

The EXT4_IOC_GET_ES_CACHE and EXT4_IOC_PRECACHE_EXTENTS currently
invokes ext4_ext_precache() to preload the extent cache without holding
the inode's i_rwsem. This can result in stale extent cache entries when
competing with operations such as ext4_collapse_range() which calls
ext4_ext_remove_space() or ext4_ext_shift_extents().

The problem arises when ext4_ext_remove_space() temporarily releases
i_data_sem due to insufficient journal credits. During this interval, a
concurrent EXT4_IOC_GET_ES_CACHE or EXT4_IOC_PRECACHE_EXTENTS may cache
extent entries that are about to be deleted. As a result, these cached
entries become stale and inconsistent with the actual extents.

Loading the extents cache without holding the inode's i_rwsem or the
mapping's invalidate_lock is not permitted besides during the writeback.
Fix this by holding the i_rwsem during EXT4_IOC_GET_ES_CACHE and
EXT4_IOC_PRECACHE_EXTENTS.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20250423085257.122685-6-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

Based on my comprehensive analysis of this commit and comparison with
the historical similar commits, here is my assessment: **YES** This
commit should be backported to stable kernel trees for the following
reasons: ## Analysis Summary ### **Type of Bug Fix** This commit
addresses a **race condition** that can lead to **stale extent cache
entries** in the ext4 filesystem. The race occurs between: 1.
`EXT4_IOC_GET_ES_CACHE` and `EXT4_IOC_PRECACHE_EXTENTS` operations 2.
Concurrent `ext4_collapse_range()` operations that call
`ext4_ext_remove_space()` or `ext4_ext_shift_extents()` ### **Technical
Details of the Race** The problem stems from the fact that
`ext4_ext_remove_space()` temporarily releases `i_data_sem` when journal
credits are insufficient. During this critical window: - The extent
precaching operations (`ext4_ext_precache()`) can cache extent entries -
These cached entries are about to be deleted by the ongoing
collapse/shift operation - Result: **stale, inconsistent extent cache
entries** ### **Code Changes Analysis** The fix is **minimal and
surgical**: **In `fs/ext4/extents.c` (ext4_get_es_cache function):**
```c if (fieinfo->fi_flags & FIEMAP_FLAG_CACHE) { +
inode_lock_shared(inode); error = ext4_ext_precache(inode); +
inode_unlock_shared(inode); if (error) return error; ``` **In
`fs/ext4/ioctl.c` (EXT4_IOC_PRECACHE_EXTENTS case):** ```c case
EXT4_IOC_PRECACHE_EXTENTS: - return ext4_ext_precache(inode); +{ + int
ret; + + inode_lock_shared(inode); + ret = ext4_ext_precache(inode); +
inode_unlock_shared(inode); + return ret; +} ``` ### **Why This
Qualifies for Backporting** 1. **Important Bug Fix**: Stale extent cache
entries can lead to filesystem inconsistencies and potential data
corruption scenarios. 2. **Minimal Risk**: The changes are extremely
small and contained: - Only adds proper locking around existing
operations - Uses shared locking (`inode_lock_shared`) which allows
concurrent readers - No algorithmic changes or new features 3. **Follows
Established Patterns**: Similar commits in the repository (marked with
"Backport Status: YES") show this pattern: - Similar commit #2: "ext4:
fix data races when using cached status extents" - also deals with
extent cache consistency - Similar commit #3: "ext4: convert to
exclusive lock while inserting delalloc extents" - addresses similar
race conditions with proper locking - Similar commit #4: "ext4: do not
polute the extents cache while shifting extents" - prevents extent cache
corruption during shift operations 4. **Clear Impact**: The commit
message explicitly states this prevents "stale extent cache entries"
which can cause inconsistency with actual extents. 5. **Contained to
Subsystem**: Changes are limited to ext4 extent management and don't
affect other kernel subsystems. 6. **Stable Backport Indicators**: - No
"Cc: stable@" tag is present, but the nature of the fix (race condition
leading to corruption) makes it a good candidate - The fix follows
ext4's established locking protocol requirements - Multiple similar
extent cache race condition fixes have been backported ### **Risk
Assessment** - **Very Low Regression Risk**: Only adds locking
protection, doesn't change functional behavior - **Well-understood
Fix**: The locking pattern matches established ext4 conventions -
**Limited Scope**: Only affects two specific ioctl code paths This
commit represents exactly the type of fix that stable trees are meant
for: a small, safe fix for a real bug that can cause filesystem
corruption without introducing new functionality or architectural
changes.

 fs/ext4/extents.c | 2 ++
 fs/ext4/ioctl.c   | 8 +++++++-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 4d7729321c80e..b805daa94206b 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -5045,7 +5045,9 @@ int ext4_get_es_cache(struct inode *inode, struct fiemap_extent_info *fieinfo,
 	}
 
 	if (fieinfo->fi_flags & FIEMAP_FLAG_CACHE) {
+		inode_lock_shared(inode);
 		error = ext4_ext_precache(inode);
+		inode_unlock_shared(inode);
 		if (error)
 			return error;
 		fieinfo->fi_flags &= ~FIEMAP_FLAG_CACHE;
diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index 1c77400bd88e1..c87094357e3d7 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -1511,8 +1511,14 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 		return 0;
 	}
 	case EXT4_IOC_PRECACHE_EXTENTS:
-		return ext4_ext_precache(inode);
+	{
+		int ret;
 
+		inode_lock_shared(inode);
+		ret = ext4_ext_precache(inode);
+		inode_unlock_shared(inode);
+		return ret;
+	}
 	case FS_IOC_SET_ENCRYPTION_POLICY:
 		if (!ext4_has_feature_encrypt(sb))
 			return -EOPNOTSUPP;
-- 
2.39.5