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 available, 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 --- Changes since v1: - Fix checkpatch error and warning.

--- tools/testing/selftests/rseq/rseq-arm.h | 52 +++++++++++++++++++++++++++++++-- 1 file changed, 50 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/rseq/rseq-arm.h b/tools/testing/selftests/rseq/rseq-arm.h index 5f262c54364f..84f28f147fb6 100644 --- a/tools/testing/selftests/rseq/rseq-arm.h +++ b/tools/testing/selftests/rseq/rseq-arm.h @@ -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"

----- On Jun 6, 2019, at 8:02 PM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:

----- On May 3, 2019, at 3:38 PM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:

...

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 available, 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.

And of course it cannot be that easy. This breaks when building in thumb mode (-mthumb). Output from librseq arm32 build [1] (code similar to what is found in the rseq selftests):

CC rseq.lo /tmp/ccu6Jw1b.s: Assembler messages: /tmp/ccu6Jw1b.s:297: Error: cannot determine Thumb instruction size. Use .inst.n/.inst.w instead /tmp/ccu6Jw1b.s:490: Error: cannot determine Thumb instruction size. Use .inst.n/.inst.w instead Makefile:460: recipe for target 'rseq.lo' failed

This appears to be caused by a missing .arm directive in RSEQ_SIG_DATA. Fixing with:

•           asm volatile ("b 2f\n\t"                                \
  

•           asm volatile (".arm\n\t"                                \
  

•                         "b 2f\n\t"                                \
  
  

gets the build to go further, but breaks at:

CC basic_percpu_ops_test.o /tmp/ccpHOMHZ.s: Assembler messages: /tmp/ccpHOMHZ.s:148: Error: misaligned branch destination /tmp/ccpHOMHZ.s:956: Error: misaligned branch destination Makefile:378: recipe for target 'basic_percpu_ops_test.o' failed

I suspect it's caused by the change from:

•           ".word " __rseq_str(RSEQ_SIG) "\n\t"                    \
  
  

to

•           ".arm\n\t"                                              \
  

•           ".inst " __rseq_str(RSEQ_SIG_CODE) "\n\t"               \
  
  

which changes the mode from thumb to arm for the rest of the inline asm within __RSEQ_ASM_DEFINE_ABORT. Better yet, there appears to be no way to save the arm/thumb state and restore it afterwards.

I'm really starting to wonder if we should go our of our way to try to get this signature to be a valid instruction on arm32. Perhaps we should consider going back to use ".word" on arm32 so it ensures it uses data endianness (which matches the parameter received by the sys_rseq system call), let objdump and friends print it as a literal pool (which it is), and just choose an instruction which has little chances to appear for the cases we care about between ARM32 BE, LE and THUMB. Perhaps a 32-bit palindrome ? Bonus points if this is a trap instruction in common configurations for odd-cases-debugging purposes.

So I'm not particularly proud of the result, but I found a rather ugly way to figure out if we are currently in thumb mode within an inline asm, and restore that mode: test the length of a nop instruction with a ".if" asm statement.

Do we want to go for this kind of approach, or should we revert back to a ".word" and accept that the rseq signature before the abort handler will be seen as data rather than an instruction on arm32 ?

Is there a better way to do this ?

Thanks,

Mathieu

diff --git a/include/rseq/rseq-arm.h b/include/rseq/rseq-arm.h index 1ce9231..b6c36dd 100644 --- a/include/rseq/rseq-arm.h +++ b/include/rseq/rseq-arm.h @@ -43,7 +43,14 @@ ({ \ int sig; \ asm volatile ("b 2f\n\t" \ + "3:\n\t" \ + "nop\n\t" \ + "4:\n\t" \ + ".arm\n\t" \ "1: .inst " __rseq_str(RSEQ_SIG_CODE) "\n\t" \ + ".if ((4b - 3b) == 2)\n\t" \ + ".thumb\n\t" \ + ".endif\n\t" \ "2:\n\t" \ "ldr %[sig], 1b\n\t" \ : [sig] "=r" (sig)); \ @@ -125,8 +132,14 @@ 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" \ + "333:\n\t" \ + "nop\n\t" \ + "444:\n\t" \ ".arm\n\t" \ ".inst " __rseq_str(RSEQ_SIG_CODE) "\n\t" \ + ".if ((444b - 333b) == 2)\n\t" \ + ".thumb\n\t" \ + ".endif\n\t" \ __rseq_str(label) ":\n\t" \ teardown \ "b %l[" __rseq_str(abort_label) "]\n\t"