mpol_to_str revisited.

[Dave Jones]

Last month I sent in 80de7c3138ee9fd86a98696fd2cf7ad89b995d0a to remove
a user triggerable BUG in mempolicy.

Ben Hutchings pointed out to me that my change introduced a potential leak
of stack contents to userspace, because none of the callers check the return value.

This patch adds the missing return checking, and also clears the buffer beforehand.

Reported-by: Ben Hutchings <bhutchings@xxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxx
Signed-off-by: Dave Jones <davej@xxxxxxxxxx>

--- 
unanswered question: why are the buffer sizes here different ? which is correct?


diff -durpN '--exclude-from=/home/davej/.exclude' src/git-trees/kernel/linux/fs/proc/task_mmu.c linux-dj/fs/proc/task_mmu.c
--- src/git-trees/kernel/linux/fs/proc/task_mmu.c	2012-05-31 22:32:46.778150675 -0400
+++ linux-dj/fs/proc/task_mmu.c	2012-10-04 19:31:41.269988984 -0400
@@ -1162,6 +1162,7 @@ static int show_numa_map(struct seq_file
 	struct mm_walk walk = {};
 	struct mempolicy *pol;
 	int n;
+	int ret;
 	char buffer[50];
 
 	if (!mm)
@@ -1178,7 +1179,11 @@ static int show_numa_map(struct seq_file
 	walk.mm = mm;
 
 	pol = get_vma_policy(proc_priv->task, vma, vma->vm_start);
-	mpol_to_str(buffer, sizeof(buffer), pol, 0);
+	memset(buffer, 0, sizeof(buffer));
+	ret = mpol_to_str(buffer, sizeof(buffer), pol, 0);
+	if (ret < 0)
+		return 0;
+
 	mpol_cond_put(pol);
 
 	seq_printf(m, "%08lx %s", vma->vm_start, buffer);
diff -durpN '--exclude-from=/home/davej/.exclude' src/git-trees/kernel/linux/mm/shmem.c linux-dj/mm/shmem.c
--- src/git-trees/kernel/linux/mm/shmem.c	2012-10-02 15:49:51.977277944 -0400
+++ linux-dj/mm/shmem.c	2012-10-04 19:32:28.862949907 -0400
@@ -885,13 +885,15 @@ redirty:
 static void shmem_show_mpol(struct seq_file *seq, struct mempolicy *mpol)
 {
 	char buffer[64];
+	int ret;
 
 	if (!mpol || mpol->mode == MPOL_DEFAULT)
 		return;		/* show nothing */
 
-	mpol_to_str(buffer, sizeof(buffer), mpol, 1);
-
-	seq_printf(seq, ",mpol=%s", buffer);
+	memset(buffer, 0, sizeof(buffer));
+	ret = mpol_to_str(buffer, sizeof(buffer), mpol, 1);
+	if (ret > 0)
+		seq_printf(seq, ",mpol=%s", buffer);
 }
 
 static struct mempolicy *shmem_get_sbmpol(struct shmem_sb_info *sbinfo)
--
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