[PATCH 3.16 044/328] mtdchar: fix overflows in adjustment of `count`

Ben Hutchings
2018-12-09T22:05:52Z

From: Jann Horn <jannh@google.com>

commit 6c6bc9ea84d0008024606bf5ba10519e20d851bf upstream.

The first checks in mtdchar_read() and mtdchar_write() attempt to limit
`count` such that `*ppos + count <= mtd->size`. However, they ignore the
possibility of `*ppos > mtd->size`, allowing the calculation of `count` to
wrap around. `mtdchar_lseek()` prevents seeking beyond mtd->size, but the
pread/pwrite syscalls bypass this.

I haven't found any codepath on which this actually causes dangerous
behavior, but it seems like a sensible change anyway.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/mtd/mtdchar.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

--- a/drivers/mtd/mtdchar.c
+++ b/drivers/mtd/mtdchar.c
@@ -190,8 +190,12 @@ static ssize_t mtdchar_read(struct file

pr_debug("MTD_read\n");

- if (*ppos + count > mtd->size)
- count = mtd->size - *ppos;
+ if (*ppos + count > mtd->size) {
+ if (*ppos < mtd->size)
+ count = mtd->size - *ppos;
+ else
+ count = 0;
+ }

if (!count)
return 0;
@@ -276,7 +280,7 @@ static ssize_t mtdchar_write(struct file

pr_debug("MTD_write\n");

- if (*ppos == mtd->size)
+ if (*ppos >= mtd->size)
return -ENOSPC;

if (*ppos + count > mtd->size)