[PATCH net] KEYS: DNS: fix parsing multiple options

[David Howells]

From: Eric Biggers <ebiggers@xxxxxxxxxx>

My recent fix for dns_resolver_preparse() printing very long strings was
incomplete, as shown by syzbot which still managed to hit the
WARN_ONCE() in set_precision() by adding a crafted "dns_resolver" key:

    precision 50001 too large
    WARNING: CPU: 7 PID: 864 at lib/vsprintf.c:2164 vsnprintf+0x48a/0x5a0

The bug this time isn't just a printing bug, but also a logical error
when multiple options ("#"-separated strings) are given in the key
payload.  Specifically, when separating an option string into name and
value, if there is no value then the name is incorrectly considered to
end at the end of the key payload, rather than the end of the current
option.  This bypasses validation of the option length, and also means
that specifying multiple options is broken -- which presumably has gone
unnoticed as there is currently only one valid option anyway.

Fix it by correctly calculating the length of the option name.

Reproducer:

    perl -e 'print "#A#", "\x00" x 50000' | keyctl padd dns_resolver desc @s

Fixes: 4a2d789267e0 ("DNS: If the DNS server returns an error, allow that to be cached [ver #2]")
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---

 net/dns_resolver/dns_key.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/dns_resolver/dns_key.c b/net/dns_resolver/dns_key.c
index 40c851693f77..d448823d4d2e 100644
--- a/net/dns_resolver/dns_key.c
+++ b/net/dns_resolver/dns_key.c
@@ -97,7 +97,7 @@ dns_resolver_preparse(struct key_preparsed_payload *prep)
 				return -EINVAL;
 			}
 
-			eq = memchr(opt, '=', opt_len) ?: end;
+			eq = memchr(opt, '=', opt_len) ?: next_opt;
 			opt_nlen = eq - opt;
 			eq++;
 			opt_vlen = next_opt - eq; /* will be -1 if no value */