Re: [musl] Erroneous rejection of pointers in __dns_parse

[Rich Felker <dalias@libc.org>]

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On Sun, Jul 16, 2023 at 08:58:04AM +0200, Markus Wichmann wrote:
> Hi all,
> 
> __dns_parse() must skip over all domain names in the package as part of
> its operation, and it also checks if the domain names end in a pointer,
> and the pointer has an offset larger than 510, because then it also
> returns failure immediately. That is probably from before the TCP merge,
> when the response buffer was a fixed 512 bytes. Now it is 768, so
> pointers can have an offset of up to 766. Except they cannot have an
> offset larger than rlen-2 in any case.

Following commit 12590c8bbd04ea484cee86812e2258fbdfca0e59, does the
attached fix seem ok?

> I am not quite sure what the point of invalid pointer detection in
> __dns_parse() is, given that if the name ever actually matters,
> __dn_expand() will reject it in its operation. But the hardcoded limit
> in __dns_parse() means that packages from TCP cannot contain pointers
> that reference the last third of the buffer.
> 
> On a related note, I see that a malformed packet can send __dn_expand()
> into an infinite loop: If a pointer points to another pointer, they can
> form a loop. The loop can be arbitrarily complex, so history tracking
> would do no good. I think it would be a good idea to reject pointers to
> pointers in that function. Because then every pointer must cause at
> least two bytes to be written to the destination buffer, so it would be
> exhausted at some point, and that's also an abort condition.

The comment on line 11 indicates how the loop is precluded. Do you
think it's incorrect?

Rich

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diff --git a/src/network/dns_parse.c b/src/network/dns_parse.c
index 7f83e791..ea1ec126 100644
--- a/src/network/dns_parse.c
+++ b/src/network/dns_parse.c
@@ -15,13 +15,13 @@ int __dns_parse(const unsigned char *r, int rlen, int (*callback)(void *, int, c
 	if (qdcount+ancount > 64) return -1;
 	while (qdcount--) {
 		while (p-r < rlen && *p-1U < 127) p++;
-		if (p>r+rlen-6 || *p>193 || (*p==193 && p[1]>254))
+		if (p>r+rlen-6)
 			return -1;
 		p += 5 + !!*p;
 	}
 	while (ancount--) {
 		while (p-r < rlen && *p-1U < 127) p++;
-		if (p>r+rlen-12 || *p>193 || (*p==193 && p[1]>254))
+		if (p>r+rlen-12)
 			return -1;
 		p += 1 + !!*p;
 		len = p[8]*256 + p[9];