@@ -5,7 +5,54 @@
* (C) Copyright 2016-2018 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
*/
-#define RSEQ_SIG 0x53053053
+/*
+ * RSEQ_SIG uses the udf A32 instruction with an uncommon immediate operand
+ * value 0x5de3. This traps if user-space reaches this instruction by mistake,
+ * and the uncommon operand ensures the kernel does not move the instruction
+ * pointer to attacker-controlled code on rseq abort.
+ *
+ * The instruction pattern in the A32 instruction set is:
+ *
+ * e7f5def3 udf #24035 ; 0x5de3
+ *
+ * This translates to the following instruction pattern in the T16 instruction
+ * set:
+ *
+ * little endian:
+ * def3 udf #243 ; 0xf3
+ * e7f5 b.n <7f5>
+ *
+ * pre-ARMv6 big endian code:
+ * e7f5 b.n <7f5>
+ * def3 udf #243 ; 0xf3
+ *
+ * ARMv6+ -mbig-endian generates mixed endianness code vs data: little-endian
+ * code and big-endian data. Ensure the RSEQ_SIG data signature matches code
+ * endianness. Prior to ARMv6, -mbig-endian generates big-endian code and data
+ * (which match), so there is no need to reverse the endianness of the data
+ * representation of the signature. However, the choice between BE32 and BE8
+ * is done by the linker, so we cannot know whether code and data endianness
+ * will be mixed before the linker is invoked.
+ */
+
+#define RSEQ_SIG_CODE 0xe7f5def3
+
+#ifndef __ASSEMBLER__
+
+#define RSEQ_SIG_DATA \
+ ({ \
+ int sig; \
+ asm volatile ( "b 2f\n\t" \
+ "1: .inst " __rseq_str(RSEQ_SIG_CODE) "\n\t" \
+ "2:\n\t" \
+ "ldr %[sig], 1b\n\t" \
+ : [sig] "=r" (sig)); \
+ sig; \
+ })
+
+#define RSEQ_SIG RSEQ_SIG_DATA
+
+#endif
#define rseq_smp_mb() __asm__ __volatile__ ("dmb" ::: "memory", "cc")
#define rseq_smp_rmb() __asm__ __volatile__ ("dmb" ::: "memory", "cc")
@@ -78,7 +125,8 @@ do { \
__rseq_str(table_label) ":\n\t" \
".word " __rseq_str(version) ", " __rseq_str(flags) "\n\t" \
".word " __rseq_str(start_ip) ", 0x0, " __rseq_str(post_commit_offset) ", 0x0, " __rseq_str(abort_ip) ", 0x0\n\t" \
- ".word " __rseq_str(RSEQ_SIG) "\n\t" \
+ ".arm\n\t" \
+ ".inst " __rseq_str(RSEQ_SIG_CODE) "\n\t" \
__rseq_str(label) ":\n\t" \
teardown \
"b %l[" __rseq_str(abort_label) "]\n\t"
Use udf as the guard instruction for the restartable sequence abort handler. Previously, the chosen signature was not a valid instruction, based on the assumption that it could always sit in a literal pool. However, there are compilation environments in which literal pools are not availble, for instance execute-only code. Therefore, we need to choose a signature value that is also a valid instruction. Handle compiling with -mbig-endian on ARMv6+, which generates binaries with mixed code vs data endianness (little endian code, big endian data). Else mismatch between code endianness for the generated signatures and data endianness for the RSEQ_SIG parameter passed to the rseq registration will trigger application segmentation faults when the kernel try to abort rseq critical sections. Prior to ARMv6, -mbig-endian generates big-endian code and data, so endianness should not be reversed in that case. Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> CC: Peter Zijlstra <peterz@infradead.org> CC: Thomas Gleixner <tglx@linutronix.de> CC: Joel Fernandes <joelaf@google.com> CC: Catalin Marinas <catalin.marinas@arm.com> CC: Dave Watson <davejwatson@fb.com> CC: Will Deacon <will.deacon@arm.com> CC: Shuah Khan <shuah@kernel.org> CC: Andi Kleen <andi@firstfloor.org> CC: linux-kselftest@vger.kernel.org CC: "H . Peter Anvin" <hpa@zytor.com> CC: Chris Lameter <cl@linux.com> CC: Russell King <linux@arm.linux.org.uk> CC: Michael Kerrisk <mtk.manpages@gmail.com> CC: "Paul E . McKenney" <paulmck@linux.vnet.ibm.com> CC: Paul Turner <pjt@google.com> CC: Boqun Feng <boqun.feng@gmail.com> CC: Josh Triplett <josh@joshtriplett.org> CC: Steven Rostedt <rostedt@goodmis.org> CC: Ben Maurer <bmaurer@fb.com> CC: linux-api@vger.kernel.org CC: Andy Lutomirski <luto@amacapital.net> CC: Andrew Morton <akpm@linux-foundation.org> CC: Linus Torvalds <torvalds@linux-foundation.org> --- tools/testing/selftests/rseq/rseq-arm.h | 52 +++++++++++++++++++++++++++++++-- 1 file changed, 50 insertions(+), 2 deletions(-)