bpf: do not use reciprocal divide

[ Upstream commit aee636c4809fa54848ff07a899b326eb1f9987a2 ]

At first Jakub Zawadzki noticed that some divisions by reciprocal_divide
were not correct. (off by one in some cases)
http://www.wireshark.org/~darkjames/reciprocal-buggy.c

He could also show this with BPF:
http://www.wireshark.org/~darkjames/set-and-dump-filter-k-bug.c

The reciprocal divide in linux kernel is not generic enough,
lets remove its use in BPF, as it is not worth the pain with
current cpus.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Jakub Zawadzki <darkjames-ws@darkjames.pl>
Cc: Mircea Gherzan <mgherzan@gmail.com>
Cc: Daniel Borkmann <dxchgb@gmail.com>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Matt Evans <matt@ozlabs.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 2 insertions, 28 deletions

diff --git a/net/core/filter.c b/net/core/filter.c
index 6438f29ff266..52f01229ee01 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -36,7 +36,6 @@
 #include <asm/uaccess.h>
 #include <asm/unaligned.h>
 #include <linux/filter.h>
-#include <linux/reciprocal_div.h>
 #include <linux/ratelimit.h>
 #include <linux/seccomp.h>
 #include <linux/if_vlan.h>
@@ -166,7 +165,7 @@ unsigned int sk_run_filter(const struct sk_buff *skb,
                         A /= X;
                         continue;
                 case BPF_S_ALU_DIV_K:
-                        A = reciprocal_divide(A, K);
+                        A /= K;
                         continue;
                 case BPF_S_ALU_MOD_X:
                         if (X == 0)
@@ -553,11 +552,6 @@ int sk_chk_filter(struct sock_filter *filter, unsigned int flen)
                 /* Some instructions need special checks */
                 switch (code) {
                 case BPF_S_ALU_DIV_K:
-                        /* check for division by zero */
-                        if (ftest->k == 0)
-                                return -EINVAL;
-                        ftest->k = reciprocal_value(ftest->k);
-                        break;
                 case BPF_S_ALU_MOD_K:
                         /* check for division by zero */
                         if (ftest->k == 0)
@@ -853,27 +847,7 @@ void sk_decode_filter(struct sock_filter *filt, struct sock_filter *to)
         to->code = decodes[code];
         to->jt = filt->jt;
         to->jf = filt->jf;
-
-        if (code == BPF_S_ALU_DIV_K) {
-                /*
-                 * When loaded this rule user gave us X, which was
-                 * translated into R = r(X). Now we calculate the
-                 * RR = r(R) and report it back. If next time this
-                 * value is loaded and RRR = r(RR) is calculated
-                 * then the R == RRR will be true.
-                 *
-                 * One exception. X == 1 translates into R == 0 and
-                 * we can't calculate RR out of it with r().
-                 */
-
-                if (filt->k == 0)
-                        to->k = 1;
-                else
-                        to->k = reciprocal_value(filt->k);
-
-                BUG_ON(reciprocal_value(to->k) != filt->k);
-        } else
-                to->k = filt->k;
+        to->k = filt->k;
 }
 
 int sk_get_filter(struct sock *sk, struct sock_filter __user *ubuf, unsigned int len)