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

+cc linux-api

On Sat, Jul 7, 2018 at 10:44 AM Boris Brezillon
[off-list ref] wrote:

On Sat,  7 Jul 2018 05:37:22 +0200
Jann Horn [off-list ref] wrote:

quoted

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>
---
 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;
+     }

Hm, shouldn't we return -ERANGE or -EINVAL if *ppos >= mtd->size?

Hmm, good question.
The pread() manpage says that pread() can return the same errors as
lseek(), and the lseek() manpage says that -EINVAL is for when you're
trying to seek beyond the end of a seekable device. So from the
documentation, it sounds as if you're right.
But testing pread() beyond end of file for various things on my
machine seems to just return 0:

# cat pread.c
#include <unistd.h>
#include <stdlib.h>
int main(int argc, char **argv) {
  char buf[0x1000];
  off_t off = strtoul(argv[1], NULL, 0);
  pread(0, buf, 0x1000, off);
}
# gcc -o pread pread.c
# strace -e trace=pread64 ./pread 200000000 < /dev/sda1
pread64(0, "", 4096, 200000000)         = 0

+++ exited with 0 +++

# strace -e trace=pread64 ./pread 100000 <
/sys/kernel/debug/x86/tlb_single_page_flush_ceiling
pread64(0, "", 4096, 100000)            = 0

+++ exited with 0 +++

# strace -e trace=pread64 ./pread 20000000000 < /dev/dm-2
pread64(0, "", 4096, 20000000000)       = 0

+++ exited with 0 +++

Do you know of precedent for returning -EINVAL if *ppos is beyond the
end of the device?

quoted

      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)

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