Messages in this thread | | | From | Kees Cook <> | Date | Wed, 7 Mar 2018 15:59:27 -0800 | Subject | Re: [PATCH] vsprintf: Remove accidental VLA usage |
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On Wed, Mar 7, 2018 at 3:42 PM, Tycho Andersen <tycho@tycho.ws> wrote: > Hi Kees, > > On Wed, Mar 07, 2018 at 03:07:14PM -0800, Kees Cook wrote: >> The "sym" calculation is actually a fixed size, but since the max() >> macro uses some extensive tricks for safety, it ends up looking like a >> variable size. This replaces max() with a simple max macro which is >> sufficient for the calculation of the array size. >> >> Seen with -Wvla. Fixed as part of the directive to remove all VLAs from >> the kernel: https://lkml.org/lkml/2018/3/7/621 >> >> Signed-off-by: Kees Cook <keescook@chromium.org> >> --- >> lib/vsprintf.c | 5 +++-- >> 1 file changed, 3 insertions(+), 2 deletions(-) >> >> diff --git a/lib/vsprintf.c b/lib/vsprintf.c >> index d7a708f82559..f420ab1477cb 100644 >> --- a/lib/vsprintf.c >> +++ b/lib/vsprintf.c >> @@ -744,8 +744,9 @@ char *resource_string(char *buf, char *end, struct resource *res, >> #define FLAG_BUF_SIZE (2 * sizeof(res->flags)) >> #define DECODED_BUF_SIZE sizeof("[mem - 64bit pref window disabled]") >> #define RAW_BUF_SIZE sizeof("[mem - flags 0x]") >> - char sym[max(2*RSRC_BUF_SIZE + DECODED_BUF_SIZE, >> - 2*RSRC_BUF_SIZE + FLAG_BUF_SIZE + RAW_BUF_SIZE)]; >> +#define SIMPLE_MAX(x, y) ((x) > (y) ? (x) : (y)) > > It's probably worth hoisting this out into some other header. When I > was looking at this a while ago, this problem happens in a few places, > see e.g. net/ipv4/proc.c:TCPUDP_MIB_MAX.
Hmm, good point. All of those suffer from the same "max*() is too fancy".
I didn't want to encourage a global macro that _lacked_ the safety built into the max*() family, though... thoughts for a reasonable approach?
-Kees
-- Kees Cook Pixel Security
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