[PATCH 1/1] lib/zlib: use atomic GCOV counters to prevent crash in inflate_fast

Konstantin Khorenko
2026-03-30T14:33:06Z

GCC's GCOV instrumentation can merge global branch counters with loop
induction variables as an optimization. In inflate_fast(), the inner
copy loops can be transformed so that GCOV counter values participate
in computing loop addresses and bounds. Since GCOV counters are global
(not per-CPU), concurrent execution on different CPUs causes the counter
to change mid-computation, producing inconsistent address calculations
and out-of-bounds memory writes.

The crash manifests during IPComp (IP Payload Compression) processing
when inflate_fast() runs concurrently on multiple CPUs:

BUG: unable to handle page fault for address: ffffd0a3c0902ffa
RIP: inflate_fast+1431
Call Trace:
zlib_inflate
__deflate_decompress
crypto_comp_decompress
ipcomp_decompress [xfrm_ipcomp]
ipcomp_input [xfrm_ipcomp]
xfrm_input

In one observed case, the compiler merged a global GCOV counter with the
loop induction variable that also indexed stores. Another CPU modified
the counter between the setup and iteration phases, causing a write
3.4 MB past the end of a 65 KB buffer.

The kernel already uses -fno-tree-loop-im for GCOV builds (commit
2b40e1ea76d4) to prevent a different optimization issue. That flag
prevents GCC from hoisting loop-invariant memory operations but does
NOT prevent the IVopts pass from merging counters with induction
variables.

Add -fprofile-update=atomic to zlib Makefiles. This tells GCC that
GCOV counters may be concurrently accessed, causing counter updates to
use atomic instructions (lock addq) instead of plain load/store.
This prevents the compiler from merging counters with loop induction
variables. The flag is scoped to zlib only to minimize performance
overhead from atomic operations in the rest of the kernel.

Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
Reviewed-by: Vasileios Almpanis <vasileios.almpanis@virtuozzo.com>
Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
---
lib/zlib_deflate/Makefile | 6 ++++++
lib/zlib_dfltcc/Makefile | 6 ++++++
lib/zlib_inflate/Makefile | 7 +++++++
3 files changed, 19 insertions(+)

diff --git a/lib/zlib_deflate/Makefile b/lib/zlib_deflate/Makefile
index 2622e03c0b942..dc0b3e5660e9e 100644
--- a/lib/zlib_deflate/Makefile
+++ b/lib/zlib_deflate/Makefile
@@ -7,6 +7,12 @@
# decompression code.
#

+# Force atomic GCOV counter updates to prevent GCC from merging global
+# counters with loop induction variables (see lib/zlib_inflate/Makefile).
+ifdef CONFIG_GCOV_KERNEL
+ccflags-y += -fprofile-update=atomic
+endif
+
obj-$(CONFIG_ZLIB_DEFLATE) += zlib_deflate.o

zlib_deflate-objs := deflate.o deftree.o deflate_syms.o
diff --git a/lib/zlib_dfltcc/Makefile b/lib/zlib_dfltcc/Makefile
index 66e1c96387c40..fb08749d2ee7b 100644
--- a/lib/zlib_dfltcc/Makefile
+++ b/lib/zlib_dfltcc/Makefile
@@ -6,6 +6,12 @@
# This is the code for s390 zlib hardware support.
#

+# Force atomic GCOV counter updates to prevent GCC from merging global
+# counters with loop induction variables (see lib/zlib_inflate/Makefile).
+ifdef CONFIG_GCOV_KERNEL
+ccflags-y += -fprofile-update=atomic
+endif
+
obj-$(CONFIG_ZLIB_DFLTCC) += zlib_dfltcc.o

zlib_dfltcc-objs := dfltcc.o dfltcc_deflate.o dfltcc_inflate.o
diff --git a/lib/zlib_inflate/Makefile b/lib/zlib_inflate/Makefile
index 27327d3e9f541..8707c649adda5 100644
--- a/lib/zlib_inflate/Makefile
+++ b/lib/zlib_inflate/Makefile
@@ -14,6 +14,13 @@
# uncompression can be done without blocking on allocation).
#

+# Force atomic GCOV counter updates to prevent GCC from merging global
+# counters with loop induction variables — concurrent inflate_fast()
+# execution on multiple CPUs causes out-of-bounds writes otherwise.
+ifdef CONFIG_GCOV_KERNEL
+ccflags-y += -fprofile-update=atomic
+endif
+
obj-$(CONFIG_ZLIB_INFLATE) += zlib_inflate.o

zlib_inflate-objs := inffast.o inflate.o infutil.o \
--
2.43.0