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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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