[PATCH] mtdchar: fix overflows in adjustment of `count`

[Jann Horn]

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 <ja...@google.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 cd67c85cc87d..02389528f622 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)
-- 
2.18.0.203.gfac676dfb9-goog