FAILED: patch "[PATCH] btrfs: avoid potential out-of-bounds in btrfs_encode_fh()" failed to apply to 5.4-stable tree

The patch below does not apply to the 5.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.4.y
git checkout FETCH_HEAD
git cherry-pick -x dff4f9ff5d7f289e4545cc936362e01ed3252742
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025101644-legacy-starch-6a67@gregkh' --subject-prefix 'PATCH 5.4.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From dff4f9ff5d7f289e4545cc936362e01ed3252742 Mon Sep 17 00:00:00 2001
From: Anderson Nascimento <[email protected]>
Date: Mon, 8 Sep 2025 09:49:02 -0300
Subject: [PATCH] btrfs: avoid potential out-of-bounds in btrfs_encode_fh()

The function btrfs_encode_fh() does not properly account for the three cases it handles.

Before writing to the file handle (fh), the function only returns to the user BTRFS_FID_SIZE_NON_CONNECTABLE (5 dwords, 20 bytes) or BTRFS_FID_SIZE_CONNECTABLE (8 dwords, 32 bytes).

However, when a parent exists and the root ID of the parent and the inode are different, the function writes BTRFS_FID_SIZE_CONNECTABLE_ROOT (10 dwords, 40 bytes).

If *max_len is not large enough, this write goes out of bounds because BTRFS_FID_SIZE_CONNECTABLE_ROOT is greater than BTRFS_FID_SIZE_CONNECTABLE originally returned.

This results in an 8-byte out-of-bounds write at
fid->parent_root_objectid = parent_root_id.

A previous attempt to fix this issue was made but was lost.

https://lore.kernel.org/all/[email protected]/

Although this issue does not seem to be easily triggerable, it is a potential memory corruption bug that should be fixed. This patch resolves the issue by ensuring the function returns the appropriate size for all three cases and validates that *max_len is large enough before writing any data.

index d062ac521051..230d9326b685 100644
@@ -23,7 +23,11 @@ static int btrfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
 	int type;
 
 	if (parent && (len < BTRFS_FID_SIZE_CONNECTABLE)) {
-		*max_len = BTRFS_FID_SIZE_CONNECTABLE;
+		if (btrfs_root_id(BTRFS_I(inode)->root) !=
+		    btrfs_root_id(BTRFS_I(parent)->root))
+			*max_len = BTRFS_FID_SIZE_CONNECTABLE_ROOT;
+		else
+			*max_len = BTRFS_FID_SIZE_CONNECTABLE;
 		return FILEID_INVALID;
 	} else if (len < BTRFS_FID_SIZE_NON_CONNECTABLE) {
 		*max_len = BTRFS_FID_SIZE_NON_CONNECTABLE;

@@ -45,6 +49,8 @@ static int btrfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
 		parent_root_id = btrfs_root_id(BTRFS_I(parent)->root);
 
 		if (parent_root_id != fid->root_objectid) {
+			if (*max_len < BTRFS_FID_SIZE_CONNECTABLE_ROOT)
+				return FILEID_INVALID;
 			fid->parent_root_objectid = parent_root_id;
 			len = BTRFS_FID_SIZE_CONNECTABLE_ROOT;
 			type = FILEID_BTRFS_WITH_PARENT_ROOT;

From: Anderson Nascimento <[email protected]>

[ Upstream commit dff4f9ff5d7f289e4545cc936362e01ed3252742 ]

The function btrfs_encode_fh() does not properly account for the three cases it handles.

Before writing to the file handle (fh), the function only returns to the user BTRFS_FID_SIZE_NON_CONNECTABLE (5 dwords, 20 bytes) or BTRFS_FID_SIZE_CONNECTABLE (8 dwords, 32 bytes).

However, when a parent exists and the root ID of the parent and the inode are different, the function writes BTRFS_FID_SIZE_CONNECTABLE_ROOT (10 dwords, 40 bytes).

If *max_len is not large enough, this write goes out of bounds because BTRFS_FID_SIZE_CONNECTABLE_ROOT is greater than BTRFS_FID_SIZE_CONNECTABLE originally returned.

This results in an 8-byte out-of-bounds write at
fid->parent_root_objectid = parent_root_id.

A previous attempt to fix this issue was made but was lost.

https://lore.kernel.org/all/[email protected]/

Although this issue does not seem to be easily triggerable, it is a potential memory corruption bug that should be fixed. This patch resolves the issue by ensuring the function returns the appropriate size for all three cases and validates that *max_len is large enough before writing any data.

---
 fs/btrfs/export.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

index 08d1d456e2f0f..c244b4cc8ba11 100644
@@ -22,7 +22,11 @@ static int btrfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
 	int type;
 
 	if (parent && (len < BTRFS_FID_SIZE_CONNECTABLE)) {
-		*max_len = BTRFS_FID_SIZE_CONNECTABLE;
+		if (BTRFS_I(inode)->root->root_key.objectid !=
+		    BTRFS_I(parent)->root->root_key.objectid)
+			*max_len = BTRFS_FID_SIZE_CONNECTABLE_ROOT;
+		else
+			*max_len = BTRFS_FID_SIZE_CONNECTABLE;
 		return FILEID_INVALID;
 	} else if (len < BTRFS_FID_SIZE_NON_CONNECTABLE) {
 		*max_len = BTRFS_FID_SIZE_NON_CONNECTABLE;

@@ -44,6 +48,8 @@ static int btrfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
 		parent_root_id = BTRFS_I(parent)->root->root_key.objectid;
 
 		if (parent_root_id != fid->root_objectid) {
+			if (*max_len < BTRFS_FID_SIZE_CONNECTABLE_ROOT)
+				return FILEID_INVALID;
 			fid->parent_root_objectid = parent_root_id;
 			len = BTRFS_FID_SIZE_CONNECTABLE_ROOT;
 			type = FILEID_BTRFS_WITH_PARENT_ROOT;

2.51.0