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SubjectRe: buffer overflow in /proc/sys/sunrpc/transports
[David Wagner - Sat, Aug 30, 2008 at 10:55:51PM +0000]
| Cyrill Gorcunov wrote:
| >Index: linux-2.6.git/net/sunrpc/sysctl.c
| >===================================================================
| >--- linux-2.6.git.orig/net/sunrpc/sysctl.c 2008-07-20 11:40:14.000000000 +0400
| >+++ linux-2.6.git/net/sunrpc/sysctl.c 2008-08-30 23:05:30.000000000 +0400
| >@@ -69,6 +69,8 @@ static int proc_do_xprt(ctl_table *table
| > return -EINVAL;
| > else {
| > len = svc_print_xprts(tmpbuf, sizeof(tmpbuf));
| >+ if (*lenp < len)
| >+ return -EFAULT;
| > if (!access_ok(VERIFY_WRITE, buffer, len))
| > return -EFAULT;
| 1. Would it be better to use copy_to_user() rather than
| access_ok() followed immediately by __copy_to_user()?

Yes, thanks

| 2. Is it OK to dereference *lenp directly? Is lenp a pointer into user
| memory or kernel memory? If it points to user memory, why is it safe to
| dereference it directly? (What about TOCTTOU bugs?) Should there be
| some sparse annotations here to ensure the code is not dereferencing
| user pointers directly? Later on, proc_do_xprt() also dereferences
| *lenp and *ppos directly.

Not only proc_do_xprt do that so I think it's safe (check for NULL
on highr level I suspect).

| 3. 'len' is declared as a signed int. len will be converted to size_t
| before doing the comparison, so if len can ever be negative (e.g.,
| svc_print_xprts() returns -1 because of an error), this patch will do
| the wrong thing. Looks like the current definition of svc_print_xprts()
| won't ever do that, as that code currently stands, so at present this
| is not a bug. However from a security point of view there are benefits
| to code whose correctness is 'locally obvious', all else being equal.
| In particular this seems like a possible maintenance hazard. Would it be
| better to use type size_t for lengths like this that are never supposed
| to be negative?

thanks, I changed it to size_t and as you mentoined negative is never coming
from called svc_print_xprts.

| 4. Is proc_dostring() relevant here?

it's not possible to use for now this function (one of my patches
was quite wrong by using it :)

- Cyrill -

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