From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mathieu Desnoyers Subject: Re: [PATCH v2 for 5.2 08/12] rseq/selftests: arm: use udf instruction for RSEQ_SIG Date: Thu, 6 Jun 2019 14:02:29 -0400 (EDT) Message-ID: <1975020343.35751.1559844149532.JavaMail.zimbra@efficios.com> References: <20190429152803.7719-9-mathieu.desnoyers@efficios.com> <20190503193858.9676-1-mathieu.desnoyers@efficios.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20190503193858.9676-1-mathieu.desnoyers@efficios.com> Sender: linux-kernel-owner@vger.kernel.org To: Will Deacon , Russell King Cc: linux-kernel , linux-api , Thomas Gleixner , Peter Zijlstra , "Paul E . McKenney" , Boqun Feng , shuah , Andy Lutomirski , Dave Watson , Paul Turner , Andrew Morton , Ingo Molnar , "H. Peter Anvin" , Andi Kleen , Chris Lameter , Ben Maurer , rostedt , Josh Triplett , Linus Torvalds , Catalin Marinas , Michael Kerrisk List-Id: linux-api@vger.kernel.org ----- 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. Thoughts ? Thanks, Mathieu [1] https://github.com/compudj/librseq > > Signed-off-by: Mathieu Desnoyers > CC: Peter Zijlstra > CC: Thomas Gleixner > CC: Joel Fernandes > CC: Catalin Marinas > CC: Dave Watson > CC: Will Deacon > CC: Shuah Khan > CC: Andi Kleen > CC: linux-kselftest@vger.kernel.org > CC: "H . Peter Anvin" > CC: Chris Lameter > CC: Russell King > CC: Michael Kerrisk > CC: "Paul E . McKenney" > CC: Paul Turner > CC: Boqun Feng > CC: Josh Triplett > CC: Steven Rostedt > CC: Ben Maurer > CC: linux-api@vger.kernel.org > CC: Andy Lutomirski > CC: Andrew Morton > CC: Linus Torvalds > --- > 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 > */ > > -#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" > -- > 2.11.0 -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com