UndefinedBehaviorSanitizer crash around slirp::ip_reass()

[Philippe Mathieu-Daudé]

tag: v4.1.0-rc1

./configure --enable-sanitizers --extra-cflags=-O1

==26130==ERROR: UndefinedBehaviorSanitizer: SEGV on unknown address 0x000000000008 (pc 0x0000561ad346d588 bp 0x7fff6ee9f940 sp 0x7fff6ee9f8e8 T26130)
==26130==The signal is caused by a WRITE memory access.
==26130==Hint: address points to the zero page.
    #0 0x0000561ad346d587 in ip_deq() at slirp/src/ip_input.c:411:55
    #1 0x0000561ad346cffb in ip_reass() at slirp/src/ip_input.c:304:9
    #2 0x0000561ad346cb6f in ip_input() at slirp/src/ip_input.c:184:18

I only had access to the last packet which isn't the culprit, I'm now seeing how to log the network traffic of the guest to provide more useful information.

[Philippe Mathieu-Daudé (philmd)]

Recent libslirp patch 126c04ac (explained in e0be8043) changed ip_reass(), so this bug might be fixed.

https://gitlab.freedesktop.org/slirp/libslirp/commit/126c04ac
https://gitlab.freedesktop.org/slirp/libslirp/commit/e0be8043

[Samuel thibault (samuel-thibault)]

And

https://gitlab.freedesktop.org/slirp/libslirp/commit/d203c81b

[Philippe Mathieu-Daudé (philmd)]

I apologize for not understanding this bug was a security issue, and not insisting on it.

It has been fixed in SLiRP by "Fix use-afte-free in ip_reass() (CVE-2020-1983)":
https://gitlab.freedesktop.org/slirp/libslirp/commit/9bd6c591

And in QEMU by commit 7769c23774 "slirp: update to fix CVE-2020-1983".

[Philippe Mathieu-Daudé (philmd)]

Fixed in QEMU release v5.0.0