Re: [syzbot] [mm?] KCSAN: data-race in generic_fillattr / shmem_mknod (2)

[Tetsuo Handa]

On 2023/04/24 17:26, Dmitry Vyukov wrote:
>> HEAD commit:    457391b03803 Linux 6.3
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=13226cf0280000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=8c81c9a3d360ebcf
>> dashboard link: https://syzkaller.appspot.com/bug?extid=702361cf7e3d95758761
>> compiler:       Debian clang version 15.0.7, GNU ld (GNU Binutils for Debian) 2.35.2
> 
> I think shmem_mknod() needs to use i_size_write() to update the size.
> Writes to i_size are not assumed to be atomic throughout the kernel
> code.
> 

I don't think that using i_size_{read,write}() alone is sufficient,
for I think that i_size_{read,write}() needs data_race() annotation.

 include/linux/fs.h |   13 +++++++++++--
 mm/shmem.c         |   12 ++++++------
 2 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 21a981680856..0d067bbe3ee9 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -860,6 +860,13 @@ void filemap_invalidate_unlock_two(struct address_space *mapping1,
  * the read or for example on x86 they can be still implemented as a
  * cmpxchg8b without the need of the lock prefix). For SMP compiles
  * and 64bit archs it makes no difference if preempt is enabled or not.
+ *
+ * However, when KCSAN is enabled, CPU being capable of reading/updating
+ * naturally aligned 8 bytes of memory atomically is not sufficient for
+ * avoiding KCSAN warning, for KCSAN checks whether value has changed between
+ * before and after of a read operation. But since we don't want to introduce
+ * seqcount overhead only for suppressing KCSAN warning, tell KCSAN that data
+ * race on accessing i_size field is acceptable.
  */
 static inline loff_t i_size_read(const struct inode *inode)
 {
@@ -880,7 +887,8 @@ static inline loff_t i_size_read(const struct inode *inode)
 	preempt_enable();
 	return i_size;
 #else
-	return inode->i_size;
+	/* See comment above. */
+	return data_race(inode->i_size);
 #endif
 }
 
@@ -902,7 +910,8 @@ static inline void i_size_write(struct inode *inode, loff_t i_size)
 	inode->i_size = i_size;
 	preempt_enable();
 #else
-	inode->i_size = i_size;
+	/* See comment above. */
+	data_race(inode->i_size = i_size);
 #endif
 }
 
diff --git a/mm/shmem.c b/mm/shmem.c
index e40a08c5c6d7..a2f20297fb59 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2951,7 +2951,7 @@ shmem_mknod(struct mnt_idmap *idmap, struct inode *dir,
 			goto out_iput;
 
 		error = 0;
-		dir->i_size += BOGO_DIRENT_SIZE;
+		i_size_write(dir, i_size_read(dir) + BOGO_DIRENT_SIZE);
 		dir->i_ctime = dir->i_mtime = current_time(dir);
 		inode_inc_iversion(dir);
 		d_instantiate(dentry, inode);
@@ -3027,7 +3027,7 @@ static int shmem_link(struct dentry *old_dentry, struct inode *dir, struct dentr
 			goto out;
 	}
 
-	dir->i_size += BOGO_DIRENT_SIZE;
+	i_size_write(dir, i_size_read(dir) + BOGO_DIRENT_SIZE);
 	inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
 	inode_inc_iversion(dir);
 	inc_nlink(inode);
@@ -3045,7 +3045,7 @@ static int shmem_unlink(struct inode *dir, struct dentry *dentry)
 	if (inode->i_nlink > 1 && !S_ISDIR(inode->i_mode))
 		shmem_free_inode(inode->i_sb);
 
-	dir->i_size -= BOGO_DIRENT_SIZE;
+	i_size_write(dir, i_size_read(dir) - BOGO_DIRENT_SIZE);
 	inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
 	inode_inc_iversion(dir);
 	drop_nlink(inode);
@@ -3132,8 +3132,8 @@ static int shmem_rename2(struct mnt_idmap *idmap,
 		inc_nlink(new_dir);
 	}
 
-	old_dir->i_size -= BOGO_DIRENT_SIZE;
-	new_dir->i_size += BOGO_DIRENT_SIZE;
+	i_size_write(old_dir, i_size_read(old_dir) - BOGO_DIRENT_SIZE);
+	i_size_write(new_dir, i_size_read(new_dir) + BOGO_DIRENT_SIZE);
 	old_dir->i_ctime = old_dir->i_mtime =
 	new_dir->i_ctime = new_dir->i_mtime =
 	inode->i_ctime = current_time(old_dir);
@@ -3189,7 +3189,7 @@ static int shmem_symlink(struct mnt_idmap *idmap, struct inode *dir,
 		folio_unlock(folio);
 		folio_put(folio);
 	}
-	dir->i_size += BOGO_DIRENT_SIZE;
+	i_size_write(dir, i_size_read(dir) + BOGO_DIRENT_SIZE);
 	dir->i_ctime = dir->i_mtime = current_time(dir);
 	inode_inc_iversion(dir);
 	d_instantiate(dentry, inode);

Maybe we want i_size_add() ?

Also, there was a similar report on updating i_{ctime,mtime} to current_time()
which means that i_size is not the only field that is causing data race.
https://syzkaller.appspot.com/bug?id=067d40ab9ab23a6fa0a8156857ed54e295062a29

Hmm, where is the serialization that avoids concurrent
shmem_mknod()/shmem_mknod() or shmem_mknod()/shmem_unlink() ?
i_size_write() says "need locking around it (normally i_mutex)"...