Quick reply - hopefully shouldn't see anything bad. But should see more bug reports if we are overflowing the kernel stack and silently corrupting stuff rather than just falling over silently. So there might be more (useful) reports. -- Computer Architect | Sent from my 64-bit #ARM Powered phone > On Sep 12, 2017, at 10:24, Peter Robinson <pbrobinson@xxxxxxxxx> wrote: > >> On Mon, Sep 4, 2017 at 2:00 AM, Jon Masters <jcm@xxxxxxxxxx> wrote: >> Can you enable CONFIG_VMAP_STACK for aarch64 kernels once this lands in >> 4.14? I'd like to see any problems flagged early and often on this... > > This is now enabled. Is there anything in particular we should be > looking for here in terms of possible problems? Just useful to get a > rough idea for those on the ground. > > P > > >> -------- Forwarded Message -------- >> Subject: [PATCHv2 00/14] arm64: VMAP_STACK support >> Date: Tue, 15 Aug 2017 13:50:35 +0100 >> From: Mark Rutland <mark.rutland@xxxxxxx> >> To: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx >> CC: ard.biesheuvel@xxxxxxxxxx, catalin.marinas@xxxxxxx, >> james.morse@xxxxxxx, labbott@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, >> luto@xxxxxxxxxxxxxx, mark.rutland@xxxxxxx, matt@xxxxxxxxxxxxxxxxxxx, >> will.deacon@xxxxxxx, kernel-hardening@xxxxxxxxxxxxxxxxxx, >> keescook@xxxxxxxxxxxx >> >> Hi, >> >> Ard and I have worked together to implement vmap stack support for >> arm64. This supersedes our earlier vmap stack RFCs [0,1]. The git author >> stats are a little misleading, as I've teased parts out into smaller >> patches for review. >> >> The series is based on our stack dump rework [2,3], which can be found >> in the arm64/exception-stack branch [4] of my kernel.org repo. This >> series can be found in the arm64/vmap-stack branch [5] of the same repo. >> >> Since v1 [6]: >> * Fix typos >> * Update comments in entry assembly >> * Dump exception context (and stacks) before regs >> * Define safe adr_this_cpu for modules >> >> On arm64, there is no double-fault exception, as software saves >> exception context to the stack. An erroneous memory access taken during >> exception handling results in a data abort, as with any other erroneous >> memory access. To avoid taking these recursively, we must detect >> overflow by checking the SP before we attempt to store any context to >> the stack. Doing this efficiently requires a couple of tricks. >> >> For a naturally aligned stack, bits THREAD_SHIFT-1:0 of a valid SP may >> contain any arbitrary value: >> >> 0bXX .. 11111111111111 >> 0bXX .. 11011001011100 >> 0bXX .. 00000000000000 >> >> By aligning stacks to double their natural alignment, we know that the >> THREAD_SHIFT bit of any valid SP must be zero: >> >> 0bXX .. 0 11111111111111 >> 0bXX .. 0 11011001011100 >> 0bXX .. 0 00000000000000 >> >> ... while an overflow will result in this bit flipping, along with >> (some) other high-order bits: >> >> 0bXX .. 0 00000000000000 >> < SP -= 1 > >> 0bXX .. 1 11111111111111 >> >> ... and thus, we can detect overflows of up to THREAD_SIZE by testing >> the THREAD_SHIFT bit of the SP value. >> >> Provided we can get the SP into a general purpose register, we can >> perform this test with a single TBNZ instruction. We don't have scratch >> space to store a GPR, but we can (partially) swap the SP with a GPR >> using arithmetic to perform the test: >> >> add sp, sp, x0 // sp' = sp + x0 >> sub x0, sp, x0 // x0' = sp' - x0 = (sp + x0) - x0 = sp >> tbnz x0, #THREAD_SHIFT, overflow_handler >> sub x0, sp, x0 // sp' - x0' = (sp + x0) - sp = x0 >> sub sp, sp, x0 // sp' - x0 = (sp + x0) - x0 = sp >> >> This series implements this approach, along with the other requisite >> changes required to make this work. >> >> The SP test is performed for all exceptions, after compensating for the >> size of the exception registers, allowing the original exception context >> to be preserved in entirety. The tests themselves are folded into the >> exception vectors, minimizing their impact. >> >> To ensure that IRQ stack overflows are detected and handled, IRQ stacks >> are now dynamically allocated, with guard pages. >> >> I've given the series some light testing with LKDTM, Syzkaller, Vince >> Weaver's perf fuzzer, and a few combinations of debug options. I haven't >> compared performance of the entire series to a baseline kernel, but from >> testing so far the cost of the SP test falls in the noise for a kernel >> build workload on Cortex-A57. >> >> Many thanks to Ard for putting up with my meddling, and also to Laura, >> James, Catalin, and Will for comments and testing. >> >> Thanks, >> Mark. >> >> [0] >> http://lists.infradead.org/pipermail/linux-arm-kernel/2017-July/518368.html >> [1] >> http://lists.infradead.org/pipermail/linux-arm-kernel/2017-July/518434.html >> [2] >> http://lists.infradead.org/pipermail/linux-arm-kernel/2017-July/520705.html >> [3] >> http://lists.infradead.org/pipermail/linux-arm-kernel/2017-July/521435.html >> [4] git://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git >> arm64/exception-stack >> [5] git://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git >> arm64/vmap-stack >> [6] >> http://lists.infradead.org/pipermail/linux-arm-kernel/2017-August/524179.html >> >> Ard Biesheuvel (2): >> arm64: kernel: remove {THREAD,IRQ_STACK}_START_SP >> arm64: assembler: allow adr_this_cpu to use the stack pointer >> >> Mark Rutland (12): >> arm64: remove __die()'s stack dump >> fork: allow arch-override of VMAP stack alignment >> arm64: factor out PAGE_* and CONT_* definitions >> arm64: clean up THREAD_* definitions >> arm64: clean up irq stack definitions >> arm64: move SEGMENT_ALIGN to <asm/memory.h> >> efi/arm64: add EFI_KIMG_ALIGN >> arm64: factor out entry stack manipulation >> arm64: use an irq stack pointer >> arm64: add basic VMAP_STACK support >> arm64: add on_accessible_stack() >> arm64: add VMAP_STACK overflow detection >> >> arch/arm64/Kconfig | 1 + >> arch/arm64/include/asm/assembler.h | 8 +- >> arch/arm64/include/asm/efi.h | 8 ++ >> arch/arm64/include/asm/irq.h | 25 ------ >> arch/arm64/include/asm/memory.h | 53 +++++++++++++ >> arch/arm64/include/asm/page-def.h | 34 +++++++++ >> arch/arm64/include/asm/page.h | 12 +-- >> arch/arm64/include/asm/processor.h | 2 +- >> arch/arm64/include/asm/stacktrace.h | 60 ++++++++++++++- >> arch/arm64/include/asm/thread_info.h | 10 +-- >> arch/arm64/kernel/entry.S | 121 >> ++++++++++++++++++++++++------ >> arch/arm64/kernel/irq.c | 40 +++++++++- >> arch/arm64/kernel/ptrace.c | 1 + >> arch/arm64/kernel/smp.c | 2 +- >> arch/arm64/kernel/stacktrace.c | 7 +- >> arch/arm64/kernel/traps.c | 44 ++++++++++- >> arch/arm64/kernel/vmlinux.lds.S | 18 +---- >> drivers/firmware/efi/libstub/arm64-stub.c | 6 +- >> kernel/fork.c | 5 +- >> 19 files changed, 353 insertions(+), 104 deletions(-) >> create mode 100644 arch/arm64/include/asm/page-def.h >> >> -- >> 1.9.1 >> >> _______________________________________________ kernel mailing list -- kernel@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe send an email to kernel-leave@xxxxxxxxxxxxxxxxxxxxxxx
