Allow access to /proc/$PID/fd after setuid()

/proc/$PID/fd has r-x------ permissions, so if process does setuid(), it
will not be able to access /proc/*/fd/. This breaks fstatat() emulation
in glibc.

open("foo", O_RDONLY|O_DIRECTORY)       = 4
setuid32(65534)                         = 0
stat64("/proc/self/fd/4/bar", 0xbfafb298) = -1 EACCES (Permission denied)

Signed-off-by: Alexey Dobriyan <adobriyan@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: James Morris <jmorris@namei.org>
Cc: Chris Wright <chrisw@sous-sol.org>
Cc: Ulrich Drepper <drepper@redhat.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Acked-By: Kirill Korotaev <dev@openvz.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index ec158dd02b3a..a721acfd4fdc 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1448,10 +1448,28 @@ static const struct file_operations proc_fd_operations = {
 };
 
 /*
+ * /proc/pid/fd needs a special permission handler so that a process can still
+ * access /proc/self/fd after it has executed a setuid().
+ */
+static int proc_fd_permission(struct inode *inode, int mask,
+                                struct nameidata *nd)
+{
+        int rv;
+
+        rv = generic_permission(inode, mask, NULL);
+        if (rv == 0)
+                return 0;
+        if (task_pid(current) == proc_pid(inode))
+                rv = 0;
+        return rv;
+}
+
+/*
  * proc directories can do almost nothing..
  */
 static const struct inode_operations proc_fd_inode_operations = {
         .lookup         = proc_lookupfd,
+        .permission     = proc_fd_permission,
         .setattr        = proc_setattr,
 };