[RFC PATCH 6/6 6.6] libfs: fix infinite directory reads for offset dir

[cel at kernel.org]

From: yangerkun <yangerkun at huawei.com>

[ Upstream commit 64a7ce76fb901bf9f9c36cf5d681328fc0fd4b5a ]

After we switch tmpfs dir operations from simple_dir_operations to
simple_offset_dir_operations, every rename happened will fill new dentry
to dest dir's maple tree(&SHMEM_I(inode)->dir_offsets->mt) with a free
key starting with octx->newx_offset, and then set newx_offset equals to
free key + 1. This will lead to infinite readdir combine with rename
happened at the same time, which fail generic/736 in xfstests(detail show
as below).

1. create 5000 files(1 2 3...) under one dir
2. call readdir(man 3 readdir) once, and get one entry
3. rename(entry, "TEMPFILE"), then rename("TEMPFILE", entry)
4. loop 2~3, until readdir return nothing or we loop too many
   times(tmpfs break test with the second condition)

We choose the same logic what commit 9b378f6ad48cf ("btrfs: fix infinite
directory reads") to fix it, record the last_index when we open dir, and
do not emit the entry which index >= last_index. The file->private_data
now used in offset dir can use directly to do this, and we also update
the last_index when we llseek the dir file.

Fixes: a2e459555c5f ("shmem: stable directory offsets")
Signed-off-by: yangerkun <yangerkun at huawei.com>
Link: https://lore.kernel.org/r/20240731043835.1828697-1-yangerkun@huawei.com
Reviewed-by: Chuck Lever <chuck.lever at oracle.com>
[brauner: only update last_index after seek when offset is zero like Jan suggested]
Signed-off-by: Christian Brauner <brauner at kernel.org>
Link: https://nvd.nist.gov/vuln/detail/CVE-2024-46701
[ cel: adjusted to apply to origin/linux-6.6.y ]
Signed-off-by: Chuck Lever <chuck.lever at oracle.com>
---
 fs/libfs.c | 37 +++++++++++++++++++++++++------------
 1 file changed, 25 insertions(+), 12 deletions(-)

diff --git a/fs/libfs.c b/fs/libfs.c
index a87005c89534..b59ff0dfea1f 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -449,6 +449,14 @@ void simple_offset_destroy(struct offset_ctx *octx)
 	xa_destroy(&octx->xa);
 }
 
+static int offset_dir_open(struct inode *inode, struct file *file)
+{
+	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
+
+	file->private_data = (void *)ctx->next_offset;
+	return 0;
+}
+
 /**
  * offset_dir_llseek - Advance the read position of a directory descriptor
  * @file: an open directory whose position is to be updated
@@ -462,6 +470,9 @@ void simple_offset_destroy(struct offset_ctx *octx)
  */
 static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
 {
+	struct inode *inode = file->f_inode;
+	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
+
 	switch (whence) {
 	case SEEK_CUR:
 		offset += file->f_pos;
@@ -475,8 +486,9 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
 	}
 
 	/* In this case, ->private_data is protected by f_pos_lock */
-	file->private_data = NULL;
-	return vfs_setpos(file, offset, U32_MAX);
+	if (!offset)
+		file->private_data = (void *)ctx->next_offset;
+	return vfs_setpos(file, offset, LONG_MAX);
 }
 
 static struct dentry *offset_find_next(struct xa_state *xas)
@@ -505,7 +517,7 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
 			  inode->i_ino, fs_umode_to_dtype(inode->i_mode));
 }
 
-static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
+static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, long last_index)
 {
 	struct offset_ctx *so_ctx = inode->i_op->get_offset_ctx(inode);
 	XA_STATE(xas, &so_ctx->xa, ctx->pos);
@@ -514,17 +526,21 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
 	while (true) {
 		dentry = offset_find_next(&xas);
 		if (!dentry)
-			return ERR_PTR(-ENOENT);
+			return;
+
+		if (dentry2offset(dentry) >= last_index) {
+			dput(dentry);
+			return;
+		}
 
 		if (!offset_dir_emit(ctx, dentry)) {
 			dput(dentry);
-			break;
+			return;
 		}
 
 		dput(dentry);
 		ctx->pos = xas.xa_index + 1;
 	}
-	return NULL;
 }
 
 /**
@@ -551,22 +567,19 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
 static int offset_readdir(struct file *file, struct dir_context *ctx)
 {
 	struct dentry *dir = file->f_path.dentry;
+	long last_index = (long)file->private_data;
 
 	lockdep_assert_held(&d_inode(dir)->i_rwsem);
 
 	if (!dir_emit_dots(file, ctx))
 		return 0;
 
-	/* In this case, ->private_data is protected by f_pos_lock */
-	if (ctx->pos == DIR_OFFSET_MIN)
-		file->private_data = NULL;
-	else if (file->private_data == ERR_PTR(-ENOENT))
-		return 0;
-	file->private_data = offset_iterate_dir(d_inode(dir), ctx);
+	offset_iterate_dir(d_inode(dir), ctx, last_index);
 	return 0;
 }
 
 const struct file_operations simple_offset_dir_operations = {
+	.open		= offset_dir_open,
 	.llseek		= offset_dir_llseek,
 	.iterate_shared	= offset_readdir,
 	.read		= generic_read_dir,
-- 
2.47.0