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5146e0688d
prctl_set_auxv() copies the user vector into a stack buffer, forces
AT_NULL on the last two entries there, and then copies only len bytes into
mm->saved_auxv. Which is fine until the vector is shorter than the
buffer, because then the forced terminator sits past the end of the copy
and never lands in saved_auxv at all.
The code even says
/* Make sure the last entry is always AT_NULL */
and it does, just not in the part that gets copied.
So mm->saved_auxv keeps the stale tail from exec. Reproducing it is easy:
from a process with CAP_SYS_RESOURCE (just run it as root), call
prctl(PR_SET_MM, PR_SET_MM_AUXV, ...) with a vector that has a couple of
entries and no AT_NULL inside len (32 bytes on arm64), and then hexdump
/proc/self/auxv, or gcore the process and look at the AUXV note with
readelf -n. This is arm64, the new vector was just { AT_UID, 0x1111,
AT_GID, 0x2222 }:
idx before (from exec) after the prctl
[0] AT_SYSINFO_EHDR 0x7ed1d6e000 AT_UID 0x1111 <- new
[1] AT_MINSIGSTKSZ 0x1270 AT_GID 0x2222 <- new
[2] AT_HWCAP 0x119fff AT_HWCAP 0x119fff <- stale
[3] AT_PAGESZ 0x1000 AT_PAGESZ 0x1000 <- stale
... 16 more entries ... <- stale
[20] AT_NULL 0x0 AT_NULL 0x0
21 entries before the prctl, still 21 after: the two new ones plus all 19
left over from exec.
Every consumer walks the vector until AT_NULL, so what they get now is a
vector that never existed at exec, the head from the prctl glued onto the
tail of the old binary. gdb and crash pull the AUXV note out of coredumps
to find AT_PHDR, AT_ENTRY, AT_SYSINFO_EHDR and friends, and a mixed vector
points them at the wrong layout. /proc/<pid>/auxv and PR_GET_AUXV hand
the same mess out to live processes too. Nothing crashes, everything just
quietly reads a frankenstein auxv.
And callers that terminate their own vector hide the whole thing, which is
likely why nobody noticed since PR_SET_MM_AUXV landed in 2012. Nothing
exciting security wise either, I mean it needs CAP_SYS_RESOURCE to begin
with.
prctl_set_mm_map() right above already copies the whole buffer for exactly
this reason, so just do the same here. user_auxv is zero initialized and
only partially filled from userspace, so the rest is zeros and nothing
leaks.
Link: https://lore.kernel.org/20260809002901.32591-1-include@grrlz.net
Fixes: fe8c7f5cbf ("c/r: prctl: extend PR_SET_MM to set up more mm_struct entries")
Signed-off-by: Bradley Morgan <include@grrlz.net>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Cyrill Gorcuno <gorcunov@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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