On Mon, 1 May 2023 at 07:16, Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote: > > 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. Agree. Or better proper READ/WRITE_ONCE. data_race() is just an annotation, it does not fix the actual data race bug that is present there. I see there are lots of uses of i_size_read() in complex scenarios that involve comparisons of the size. All such racy uses are subject to the TOCTOU bug at least. > 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)"... >
