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SubjectRe: [PATCH] SUNRPC: Prevent kernel stack corruption on long values of flush
J. Bruce Fields wrote:

On Tue, Jul 17, 2012 at 12:01:26AM +0200, Sasha Levin wrote:
> The buffer size in read_flush() is too small for the longest possible values
> for it. This can lead to a kernel stack corruption:
Thanks!

>
> diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
> index 2afd2a8..f86d95e 100644
> --- a/net/sunrpc/cache.c
> +++ b/net/sunrpc/cache.c
> @@ -1409,11 +1409,11 @@ static ssize_t read_flush(struct file *file, char __user *buf,
> size_t count, loff_t *ppos,
> struct cache_detail *cd)
> {
> - char tbuf[20];
> + char tbuf[22];

I wonder how common this sort of calculation is in the kernel? It might
provide some peace of mind to be able to write this something like

char tbuf[MAXLEN_BASE10_UL + 2] /* + 2 for final "\n\0" */
You could use something like:

char tbuf[sizeof (unsigned long) * 24 / 10 + 1 + 2]; /* + 2 for final "\n\0" */
since there are roughly 10 bits for every 3 decimal digits.

But I'm obviously confused, because I don't understand why tbuf needs to be
any more than 10 + 2.


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