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KEYS: DNS: limit the length of option strings 

Adding a dns_resolver key whose payload contains a very long option name
resulted in that string being printed in full.  This hit the WARN_ONCE()
in set_precision() during the printk(), because printk() only supports a
precision of up to 32767 bytes:

    precision 1000000 too large
    WARNING: CPU: 0 PID: 752 at lib/vsprintf.c:2189 vsnprintf+0x4bc/0x5b0

Fix it by limiting option strings (combined name + value) to a much more
reasonable 128 bytes.  The exact limit is arbitrary, but currently the
only recognized option is formatted as "dnserror=%lu" which fits well
within this limit.

Also ratelimit the printks.

Reproducer:

    perl -e 'print "#", "A" x 1000000, "\x00"' | keyctl padd dns_resolver desc @s

This bug was found using syzkaller.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Fixes: 4a2d789267 ("DNS: If the DNS server returns an error, allow that to be cached [ver #2]")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Biggers 2018-04-17 12:07:06 -07:00 committed by David S. Miller
parent 89bda97b44
commit 9c438d7a3a
1 changed files with 5 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
net/dns_resolver/dns_key.c
View File

@ -91,9 +91,9 @@ dns_resolver_preparse(struct key_preparsed_payload *prep)
next_opt = memchr(opt, '#', end - opt) ?: end;
opt_len = next_opt - opt;
if (!opt_len) {
printk(KERN_WARNING
"Empty option to dns_resolver key\n");
if (opt_len <= 0 || opt_len > 128) {
pr_warn_ratelimited("Invalid option length (%d) for dns_resolver key\n",
opt_len);
return -EINVAL;
}
@ -127,10 +127,8 @@ dns_resolver_preparse(struct key_preparsed_payload *prep)
}
bad_option_value:
printk(KERN_WARNING
"Option '%*.*s' to dns_resolver key:"
" bad/missing value\n",
opt_nlen, opt_nlen, opt);
pr_warn_ratelimited("Option '%*.*s' to dns_resolver key: bad/missing value\n",
opt_nlen, opt_nlen, opt);
return -EINVAL;
} while (opt = next_opt + 1, opt < end);
}