[PATCH] do_coredump() and not stopping rewrite attacks?

On Sat, Dec 02, 2006 at 11:47:44PM +0300, Alexey Dobriyan wrote:
> David Binderman compiled 2.6.19 with icc and grepped for "was set but never
> used". Many warnings are on
> 	http://coderock.org/kj/unused-2.6.19-fs

Heh, the very first line:
fs/exec.c(1465): remark #593: variable "flag" was set but never used

fs/exec.c:
  1477		/*
  1478		 *	We cannot trust fsuid as being the "true" uid of the
  1479		 *	process nor do we know its entire history. We only know it
  1480		 *	was tainted so we dump it as root in mode 2.
  1481		 */
  1482		if (mm->dumpable == 2) {	/* Setuid core dump mode */
  1483			flag = O_EXCL;		/* Stop rewrite attacks */
  1484			current->fsuid = 0;	/* Dump root private */
  1485		}

And then filp_open follows with "flag" totally ignored.

(akpm: this restores the code to Alan's original version.  Andi's "Support
piping into commands in /proc/sys/kernel/core_pattern" (cset d025c9db) broke
it).

Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: <stable@kerenl.org>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

fs/exec.c

@@ -1515,7 +1515,8 @@ int do_coredump(long signr, int exit_code, struct pt_regs * regs)
 		ispipe = 1;
  	} else
  		file = filp_open(corename,
-				 O_CREAT | 2 | O_NOFOLLOW | O_LARGEFILE, 0600);
+				 O_CREAT | 2 | O_NOFOLLOW | O_LARGEFILE | flag,
+				 0600);
 	if (IS_ERR(file))
 		goto fail_unlock;
 	inode = file->f_dentry->d_inode;