Re: setfsuid() and access() syscall

["Serge E. Hallyn" <serue@us.ibm.com>]

Quoting Ondrej Palkovsky (ondrap@penguin.cz):
> Hello,
>
> the access() syscall (to find out if the user has permission to do  
> something on file) does not seem to reflect the setfsuid() syscall.  
> There are 2 conflicting pieces of information:
>
> - kernel/sys.c:
> /*
> * "setfsuid()" sets the fsuid - the uid used for filesystem checks. This
> * is used for "access()" and for the NFS daemon (letting nfsd stay at

Good catch that.  This comment needs to be fixed (proposed patch below).

> * whatever uid it wants to). It normally shadows "euid", except when
> * explicitly set by setfsuid() or for access..
> */
> - fs/namei.c
> /*
> * access() needs to use the real uid/gid, not the effective uid/gid.
> * We do this by temporarily clearing all FS-related capabilities and
> * switching the fsuid/fsgid around to the real ones.
> */
>
> The resulting behaviour (2.6.18, 2.6.28, source code for 2.6.30 seems to  
> be the same) seems to be that access() is dependent on uid, not fsuid -  
> this seems to me to be a bug, which unfortunately somewhat inhibits  
> multithreaded file servers that want to use access() e.g. for ACL  
> checks. Is there some reason why it is implemented the way it is as it  
> looks like an intention?
>
> Best regards
> Ondrej Palkovsky

>From d0450cb216753d8c1d2d941bb5f4e15fe7aa2caf Mon Sep 17 00:00:00 2001
From: Serge Hallyn <serue@us.ibm.com>
Date: Tue, 4 Aug 2009 16:49:46 -0500
Subject: [PATCH 1/1] fix setfsuid comment: fsuid is not used for access

Fix the comment above setfsuid which currently says that the
fsuid is used for access().  In fact, ruid is used for access.

Signed-off-by: Serge Hallyn <serue@us.ibm.com>
---
 kernel/sys.c |    8 +++++---
 1 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index b3f1097..94e6622 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -835,9 +835,11 @@ SYSCALL_DEFINE3(getresgid, gid_t __user *, rgid, gid_t __user *, egid, gid_t __u
 
 /*
  * "setfsuid()" sets the fsuid - the uid used for filesystem checks. This
- * is used for "access()" and for the NFS daemon (letting nfsd stay at
- * whatever uid it wants to). It normally shadows "euid", except when
- * explicitly set by setfsuid() or for access..
+ * is used when setting uid for a new file, for calculating file permissions,
+ * and for the NFS daemon (letting nfsd stay at whatever uid it wants to).
+ *
+ * It normally shadows "euid", except when explicitly set by setfsuid() or
+ * for access..
  */
 SYSCALL_DEFINE1(setfsuid, uid_t, uid)
 {
-- 
1.6.0.4