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locks: fix TOCTOU race when granting write lease

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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
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7
fs/file_table.c
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@@ -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);