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locks: fix TOCTOU race when granting write lease 

Thread A trying to acquire a write lease checks the value of i_readcount
and i_writecount in check_conflicting_open() to verify that its own fd
is the only fd referencing the file.

Thread B trying to open the file for read will call break_lease() in
do_dentry_open() before incrementing i_readcount, which leaves a small
window where thread A can acquire the write lease and then thread B
completes the open of the file for read without breaking the write lease
that was acquired by thread A.

Fix this race by incrementing i_readcount before checking for existing
leases, same as the case with i_writecount.

Use a helper put_file_access() to decrement i_readcount or i_writecount
in do_dentry_open() and __fput().

Fixes: 387e3746d0 ("locks: eliminate false positive conflicts for write lease")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Amir Goldstein 2022-08-16 17:53:17 +03:00 committed by Al Viro
parent 568035b01c
commit d6da19c9ca
3 changed files with 15 additions and 13 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
fs/file_table.c
View File

@ -324,12 +324,7 @@ static void __fput(struct file *file)
}
fops_put(file->f_op);
put_pid(file->f_owner.pid);
if ((mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
i_readcount_dec(inode);
if (mode & FMODE_WRITER) {
put_write_access(inode);
__mnt_drop_write(mnt);
}
put_file_access(file);
dput(dentry);
if (unlikely(mode & FMODE_NEED_UNMOUNT))
dissolve_on_fput(mnt);

10
fs/internal.h
View File

@ -101,6 +101,16 @@ extern void chroot_fs_refs(const struct path *, const struct path *);
extern struct file *alloc_empty_file(int, const struct cred *);
extern struct file *alloc_empty_file_noaccount(int, const struct cred *);
static inline void put_file_access(struct file *file)
{
if ((file->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ) {
i_readcount_dec(file->f_inode);
} else if (file->f_mode & FMODE_WRITER) {
put_write_access(file->f_inode);
__mnt_drop_write(file->f_path.mnt);
}
}
/*
* super.c
*/

11
fs/open.c
View File

@ -840,7 +840,9 @@ static int do_dentry_open(struct file *f,
return 0;
}
if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
if ((f->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ) {
i_readcount_inc(inode);
} else if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
error = get_write_access(inode);
if (unlikely(error))
goto cleanup_file;
@ -880,8 +882,6 @@ static int do_dentry_open(struct file *f,
goto cleanup_all;
}
f->f_mode |= FMODE_OPENED;
if ((f->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
i_readcount_inc(inode);
if ((f->f_mode & FMODE_READ) &&
likely(f->f_op->read || f->f_op->read_iter))
f->f_mode |= FMODE_CAN_READ;
@ -935,10 +935,7 @@ cleanup_all:
if (WARN_ON_ONCE(error > 0))
error = -EINVAL;
fops_put(f->f_op);
if (f->f_mode & FMODE_WRITER) {
put_write_access(inode);
__mnt_drop_write(f->f_path.mnt);
}
put_file_access(f);
cleanup_file:
path_put(&f->f_path);
f->f_path.mnt = NULL;