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vfs: vfs_tmpfile: ensure O_EXCL flag is enforced 

If O_EXCL is *not* specified, then linkat() can be
used to link the temporary file into the filesystem.
If O_EXCL is specified then linkat() should fail (-1).

After commit 863f144f12 ("vfs: open inside ->tmpfile()")
the O_EXCL flag is no longer honored by the vfs layer for
tmpfile, which means the file can be linked even if O_EXCL
flag is specified, which is a change in behaviour for
userspace!

The open flags was previously passed as a parameter, so it
was uneffected by the changes to file->f_flags caused by
finish_open(). This patch fixes the issue by storing
file->f_flags in a local variable so the O_EXCL test
logic is restored.

This regression was detected by Android CTS Bionic fcntl()
tests running on android-mainline [1].

[1] https://android.googlesource.com/platform/bionic/+/
    refs/heads/master/tests/fcntl_test.cpp#352

Fixes: 863f144f12 ("vfs: open inside ->tmpfile()")
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Tested-by: Will McVicker <willmcvicker@google.com>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Peter Griffin 2022-11-03 17:02:10 +00:00 committed by Al Viro
parent 7ee47dcfff
commit 406c706c7b
1 changed files with 2 additions and 1 deletions
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3
fs/namei.c
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@ -3591,6 +3591,7 @@ static int vfs_tmpfile(struct user_namespace *mnt_userns,
struct inode *dir = d_inode(parentpath->dentry);
struct inode *inode;
int error;
int open_flag = file->f_flags;
/* we want directory to be writable */
error = inode_permission(mnt_userns, dir, MAY_WRITE | MAY_EXEC);
@ -3613,7 +3614,7 @@ static int vfs_tmpfile(struct user_namespace *mnt_userns,
if (error)
return error;
inode = file_inode(file);
if (!(file->f_flags & O_EXCL)) {
if (!(open_flag & O_EXCL)) {
spin_lock(&inode->i_lock);
inode->i_state |= I_LINKABLE;
spin_unlock(&inode->i_lock);