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locks: fix TOCTOU race when granting write lease
[ Upstream commitd6da19c9ca
] 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> Signed-off-by: Sasha Levin <sashal@kernel.org>
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7e053784c4
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a29b6eb959
@ -284,12 +284,7 @@ static void __fput(struct file *file)
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}
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fops_put(file->f_op);
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put_pid(file->f_owner.pid);
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if ((mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
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i_readcount_dec(inode);
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if (mode & FMODE_WRITER) {
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put_write_access(inode);
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__mnt_drop_write(mnt);
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}
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put_file_access(file);
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dput(dentry);
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if (unlikely(mode & FMODE_NEED_UNMOUNT))
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dissolve_on_fput(mnt);
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@ -100,6 +100,16 @@ extern void chroot_fs_refs(const struct path *, const struct path *);
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extern struct file *alloc_empty_file(int, const struct cred *);
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extern struct file *alloc_empty_file_noaccount(int, const struct cred *);
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static inline void put_file_access(struct file *file)
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{
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if ((file->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ) {
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i_readcount_dec(file->f_inode);
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} else if (file->f_mode & FMODE_WRITER) {
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put_write_access(file->f_inode);
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__mnt_drop_write(file->f_path.mnt);
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}
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}
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/*
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* super.c
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*/
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11
fs/open.c
11
fs/open.c
@ -786,7 +786,9 @@ static int do_dentry_open(struct file *f,
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return 0;
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}
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if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
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if ((f->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ) {
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i_readcount_inc(inode);
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} else if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
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error = get_write_access(inode);
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if (unlikely(error))
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goto cleanup_file;
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@ -826,8 +828,6 @@ static int do_dentry_open(struct file *f,
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goto cleanup_all;
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}
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f->f_mode |= FMODE_OPENED;
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if ((f->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
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i_readcount_inc(inode);
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if ((f->f_mode & FMODE_READ) &&
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likely(f->f_op->read || f->f_op->read_iter))
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f->f_mode |= FMODE_CAN_READ;
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@ -880,10 +880,7 @@ cleanup_all:
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if (WARN_ON_ONCE(error > 0))
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error = -EINVAL;
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fops_put(f->f_op);
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if (f->f_mode & FMODE_WRITER) {
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put_write_access(inode);
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__mnt_drop_write(f->f_path.mnt);
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}
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put_file_access(f);
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cleanup_file:
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path_put(&f->f_path);
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f->f_path.mnt = NULL;
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