On Tue, 12 Nov 2024 23:38:11 +0900, Ryusuke Konishi wrote: > On Tue, Nov 12, 2024 at 7:56 PM Edward Adam Davis wrote: > > > > The i_size value of the directory "cgroup.controllers" opened by openat is 0, > > which causes 0 to be returned when calculating the last valid byte in > > nilfs_last_byte(), which ultimately causes kaddr to move forward by reclen > > (its value is 32 in this case), which ultimately triggers the uaf when > > accessing de->rec_len in nilfs_find_entry(). > > > > To avoid this issue, add a check for i_size in nilfs_lookup(). > > > > Reported-by: syzbot+96d5d14c47d97015c624@xxxxxxxxxxxxxxxxxxxxxxxxx > > Closes: https://syzkaller.appspot.com/bug?extid=96d5d14c47d97015c624 > > Signed-off-by: Edward Adam Davis <eadavis@xxxxxx> > > --- > > fs/nilfs2/namei.c | 3 +++ > > 1 file changed, 3 insertions(+) > > Hi Edward, thanks for the debugging help and patch suggestion. > > But this fix is incorrect. > > Reproducers are not creating the situation where i_size == 0. > In my debug message output inserted in the while loop of > nilfs_find_entry(), i_size was a corrupted large value like this: > > NILFS (loop0): nilfs_find_entry: isize=422212465065984, > npages=103079215104, n=0, last_byte=0, reclen=32 > > This is different from your debug result, because the type of i_size > in the debug patch you sent to syzbot is "%u". > The type of inode->i_size is "loff_t", which is "long long". > Therefore, the output format specification for i_size in the debug > output should be "%lld". Yes, you are right, I ignore the type of i_size. > > If you look at the beginning of nilfs_find_entry(), you can see that > your check is double-checked: > > struct nilfs_dir_entry *nilfs_find_entry(struct inode *dir, > const struct qstr *qstr, struct folio **foliop) > { > ... > unsigned long npages = dir_pages(dir); Yes, now I noticed dir_pages(). > .. > > if (npages == 0) > goto out; > ... > > Here, dir_pages() returns 0 if i_size is 0, so it jumps to "out" and > returns ERR_PTR(-ENOENT). > > I'm still debugging, but one problem is that the implementation of > nilfs_last_byte() is incorrect. > In the following part, the local variable "last_byte" is not of type > "loff_t", so depending on the value, it may be truncated and return a > wrong value (0 in this case): > > static unsigned int nilfs_last_byte(struct inode *inode, unsigned long page_nr) > { > unsigned int last_byte = inode->i_size; > ... > } > > If this is the only problem, the following fix will be effective. (To > complete this fix, I think we need to think more carefully about > whether it's okay for i_size to have any value, especially since > loff_t is a signed type): > > diff --git a/fs/nilfs2/dir.c b/fs/nilfs2/dir.c > index a8602729586a..6bc8f474a3e5 100644 > --- a/fs/nilfs2/dir.c > +++ b/fs/nilfs2/dir.c > @@ -70,7 +70,7 @@ static inline unsigned int nilfs_chunk_size(struct > inode *inode) > */ > static unsigned int nilfs_last_byte(struct inode *inode, unsigned long page_nr) > { > - unsigned int last_byte = inode->i_size; > + loff_t last_byte = inode->i_size; > > last_byte -= page_nr << PAGE_SHIFT; > if (last_byte > PAGE_SIZE) > I have noticed nilfs_last_byte(), I have other concerns about it, such as the chance of last_byte overflowing when i_size is too small and page_nr is too large, or that it will be negative after being type-adjusted to loff_t. So, maybe following fix is more rigorous. diff --git a/fs/nilfs2/dir.c b/fs/nilfs2/dir.c index a8602729586a..0dbcf91538fd 100644 --- a/fs/nilfs2/dir.c +++ b/fs/nilfs2/dir.c @@ -70,9 +70,10 @@ static inline unsigned int nilfs_chunk_size(struct inode *inode) */ static unsigned int nilfs_last_byte(struct inode *inode, unsigned long page_nr) { - unsigned int last_byte = inode->i_size; + loff_t last_byte = inode->i_size; - last_byte -= page_nr << PAGE_SHIFT; + if (last_byte > page_nr << PAGE_SHIFT) + last_byte -= page_nr << PAGE_SHIFT; if (last_byte > PAGE_SIZE) last_byte = PAGE_SIZE; return last_byte; BR, Edward > > Regards, > Ryusuke Konishi > > > > > > diff --git a/fs/nilfs2/namei.c b/fs/nilfs2/namei.c > > index 9b108052d9f7..0b57bcd9c2c5 100644 > > --- a/fs/nilfs2/namei.c > > +++ b/fs/nilfs2/namei.c > > @@ -60,6 +60,9 @@ nilfs_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags) > > if (dentry->d_name.len > NILFS_NAME_LEN) > > return ERR_PTR(-ENAMETOOLONG); > > > > + if (!dir->i_size) > > + return ERR_PTR(-EINVAL); > > + > > res = nilfs_inode_by_name(dir, &dentry->d_name, &ino); > > if (res) { > > if (res != -ENOENT) > > -- > > 2.43.0 > >
