netfilter: nf_conntrack_h323: fix OOB read in decode_choice()

Overview

In the Linux kernel, the following vulnerability has been resolved:

netfilter: nf_conntrack_h323: fix OOB read in decode_choice()

In decode_choice(), the boundary check before get_len() uses the variable len, which is still 0 from its initialization at the top of the function:

unsigned int type, ext, len = 0;
...
if (ext || (son->attr & OPEN)) {
    BYTE_ALIGN(bs);
    if (nf_h323_error_boundary(bs, len, 0))  /* len is 0 here */
        return H323_ERROR_BOUND;
    len = get_len(bs);                        /* OOB read */

When the bitstream is exactly consumed (bs->cur == bs->end), the check nf_h323_error_boundary(bs, 0, 0) evaluates to (bs->cur + 0 > bs->end), which is false. The subsequent get_len() call then dereferences *bs->cur++, reading 1 byte past the end of the buffer. If that byte has bit 7 set, get_len() reads a second byte as well.

This can be triggered remotely by sending a crafted Q.931 SETUP message with a User-User Information Element containing exactly 2 bytes of PER-encoded data ({0x08, 0x00}) to port 1720 through a firewall with the nf_conntrack_h323 helper active. The decoder fully consumes the PER buffer before reaching this code path, resulting in a 1-2 byte heap-buffer-overflow read confirmed by AddressSanitizer.

Fix this by checking for 2 bytes (the maximum that get_len() may read) instead of the uninitialized len. This matches the pattern used at every other get_len() call site in the same file, where the caller checks for 2 bytes of available data before calling get_len().

Affected

• Linux >= ec8a8f3c31ddef0a7d9626c4b8a4baa30f3b80aa < bcb50aa0b8f2b74a9fe5a1c7bee6f2657a288041
• Linux >= ec8a8f3c31ddef0a7d9626c4b8a4baa30f3b80aa < 2a3aac4205e7d2f1aca2e3827de8cdd517d36c4a
• Linux >= ec8a8f3c31ddef0a7d9626c4b8a4baa30f3b80aa < 81f2fc5b0d0cf4696146f00f837596d10b92dead
• Linux >= ec8a8f3c31ddef0a7d9626c4b8a4baa30f3b80aa < 7ef82863d42261817a6394c6c881bd6757a70f16
• Linux >= ec8a8f3c31ddef0a7d9626c4b8a4baa30f3b80aa < 53d32735d77ab56cc3fc7bd53a7d099418f19be1
• Linux >= ec8a8f3c31ddef0a7d9626c4b8a4baa30f3b80aa < f0a83d0a4b7c127d32ac06d607a9214937716129
• Linux >= ec8a8f3c31ddef0a7d9626c4b8a4baa30f3b80aa < 35f1943d242e1b9f0b6e91c0c93bfb293a9f8224
• Linux >= ec8a8f3c31ddef0a7d9626c4b8a4baa30f3b80aa < baed0d9ba91d4f390da12d5039128ee897253d60
• Linux 4.15

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.