[PATCH AUTOSEL 4.9 03/34] mtdchar: fix overflows in adjustment of `count`

Sasha Levin
2018-09-15T01:34:34Z

From: Jann Horn <jannh@google.com>

[ Upstream commit 6c6bc9ea84d0008024606bf5ba10519e20d851bf ]

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: Sasha Levin <alexander.levin@microsoft.com>
---
drivers/mtd/mtdchar.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/mtd/mtdchar.c b/drivers/mtd/mtdchar.c
index b4092eab53ac..95b6a6640bca 100644
--- a/drivers/mtd/mtdchar.c
+++ b/drivers/mtd/mtdchar.c
@@ -160,8 +160,12 @@ static ssize_t mtdchar_read(struct file *file, char __user *buf, size_t count,

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;
@@ -246,7 +250,7 @@ static ssize_t mtdchar_write(struct file *file, const char __user *buf, size_t c

pr_debug("MTD_write\n");

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

if (*ppos + count > mtd->size)
--
2.17.1