get_mm_cmdline() leaks an uninitialized byte located at
user-controlled offset in a newly-allocated kernel page in
the following scenario.
- When reading the last chunk of cmdline, access_remote_vm()
fails to copy the requested number of bytes, but still copies
enough bytes so that we get into the body of
"if (pos + got >= arg_end)" statement. This can be arranged by user,
for example, by applying mprotect(PROT_NONE) to the env block.
- strnlen() doesn't find a NUL byte. This too can be arranged
by user via suitable modifications of argument and env blocks.
- The above causes the following condition to be true despite
that no NUL byte was found:
/* Include the NUL if it existed */
if (got < size)
got++;
The resulting increment causes the subsequent copy_to_user()
to copy an extra byte from "page" to userspace. That byte might
come from previous uses of memory referred by "page" before
it was allocated by get_mm_cmdline(), potentially leaking
data belonging to other processes or kernel.
Fix this by ensuring that "size + offset" doesn't exceed the number
of bytes copied by access_remote_vm().
Fixes: f5b65348fd77 ("proc: fix missing final NUL in get_mm_cmdline() rewrite")
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Alexey Izbyshev <izbyshev@xxxxxxxxx>
---
This patch was initially sent to <security@xxxxxxxxxx> accompanied
with a little program that exploits the bug to dump the kernel page
used in get_mm_cmdline().
Thanks to Oleg Nesterov and Laura Abbott for their feedback!
fs/proc/base.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 255f6754c70d..6e30dd791761 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -275,6 +275,8 @@ static ssize_t get_mm_cmdline(struct mm_struct *mm, char __user *buf,
if (got <= offset)
break;
got -= offset;
+ if (got < size)
+ size = got;
/* Don't walk past a NUL character once you hit arg_end */
if (pos + got >= arg_end) {
--
2.21.0
