Red Hat Bugzilla – Bug 1463692
glibc: Incorrect pointer alignment in NSS group merge result construction
Last modified: 2018-04-10 10:01:54 EDT
Sergey Kolosov discovered that the NSS group merge feature (added in bug 1298975) does not work on ppc64 and s390x. The cause seems to be this code in grp/grp-merge.c: /* Copy the pointers from the members array into the buffer and assign them to the gr_mem member of destgrp. */ destgrp->gr_mem = (char **) &destbuf[c]; len = sizeof (char *) * (memcount + 1); BUFCHECK (len); memcpy (&destbuf[c], members, len); c += len; free (members); members = NULL; After the assigned to destgrp->gr_mem, we have: (gdb) print destbuf $1 = 0x10031000 "groupa" (gdb) print c $2 = 21 (gdb) print destgrp->gr_mem $3 = (char **) 0x10031015 This pointer is not suitable aligned, and this triggers undefined behavior. We should switch this code over to the allocation buffer interface, which deals with exactly this kind of problem.
Correct, gr_mem is not sufficiently aligned at the start. On pcc64 any * must be doubleword aligned (8 bytes). On s390 any * must be word aligned (4 bytes). These are just natural alignment rules.
Absolutely needs to be fixed and is already fixed in upstream master: commit f8cef4d07d9641e27629bd3ce2d13f5d702fb251 Author: DJ Delorie <dj@delorie.com> Date: Wed Jul 19 13:14:34 2017 -0400 Fix cast-after-dereference Original code was dereferencing a char*, then casting the value to size_t. Should cast the pointer to size_t* then deference. commit 4fa8ae49aa169fb8d97882938e8bee3ed9ce5410 Author: DJ Delorie <dj@delorie.com> Date: Fri Jul 14 21:46:42 2017 -0400 Fix BZ #21654 - grp-merge.c alignment * grp/grp_merge.c (__copy_grp): Align char** to minimum pointer alignment not char alignment. (__merge_grp): Likewise.
Note that comment 2 shows the real fix, I think the alignment wasn't the actual cause of the bug. We need all three commits, including: commit 4fa8ae49aa169fb8d97882938e8bee3ed9ce5410 Author: DJ Delorie <dj@delorie.com> Date: Fri Jul 14 21:46:42 2017 -0400 Fix BZ #21654 - grp-merge.c alignment * grp/grp_merge.c (__copy_grp): Align char** to minimum pointer alignment not char alignment. (__merge_grp): Likewise.
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory, and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHSA-2018:0805