fs/proc/task_mmu.c: fix buffer overflow in add_page_map()

Recently we met quite a lot of random kernel panic issues after enabling
CONFIG_PROC_PAGE_MONITOR.  After debuggind we found this has something
to do with following bug in pagemap:

In struct pagemapread:

  struct pagemapread {
      int pos, len;
      pagemap_entry_t *buffer;
      bool v2;
  };

pos is number of PM_ENTRY_BYTES in buffer, but len is the size of
buffer, it is a mistake to compare pos and len in add_page_map() for
checking buffer is full or not, and this can lead to buffer overflow and
random kernel panic issue.

Correct len to be total number of PM_ENTRY_BYTES in buffer.

[akpm@linux-foundation.org: document pagemapread.pos and .len units, fix PM_ENTRY_BYTES definition]
Signed-off-by: Yonghua Zheng <younghua.zheng@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 4 deletions

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index a11720767abc..107d026f5d6e 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -868,7 +868,7 @@ typedef struct {
 } pagemap_entry_t;
 
 struct pagemapread {
-        int pos, len;
+        int pos, len;           /* units: PM_ENTRY_BYTES, not bytes */
         pagemap_entry_t *buffer;
         bool v2;
 };
@@ -876,7 +876,7 @@ struct pagemapread {
 #define PAGEMAP_WALK_SIZE       (PMD_SIZE)
 #define PAGEMAP_WALK_MASK       (PMD_MASK)
 
-#define PM_ENTRY_BYTES      sizeof(u64)
+#define PM_ENTRY_BYTES      sizeof(pagemap_entry_t)
 #define PM_STATUS_BITS      3
 #define PM_STATUS_OFFSET    (64 - PM_STATUS_BITS)
 #define PM_STATUS_MASK      (((1LL << PM_STATUS_BITS) - 1) << PM_STATUS_OFFSET)
@@ -1127,8 +1127,8 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
                 goto out_task;
 
         pm.v2 = soft_dirty_cleared;
-        pm.len = PM_ENTRY_BYTES * (PAGEMAP_WALK_SIZE >> PAGE_SHIFT);
-        pm.buffer = kmalloc(pm.len, GFP_TEMPORARY);
+        pm.len = (PAGEMAP_WALK_SIZE >> PAGE_SHIFT);
+        pm.buffer = kmalloc(pm.len * PM_ENTRY_BYTES, GFP_TEMPORARY);
         ret = -ENOMEM;
         if (!pm.buffer)
                 goto out_task;