On Fri, Oct 16, 2020 at 05:12:33PM -0700, Peter Collingbourne wrote: > The kernel currently clears the tag bits (i.e. bits 56-63) in the fault > address exposed via siginfo.si_addr and sigcontext.fault_address. However, > the tag bits may be needed by tools in order to accurately diagnose > memory errors, such as HWASan [1] or future tools based on the Memory > Tagging Extension (MTE). > > We should not stop clearing these bits in the existing fault address > fields, because there may be existing userspace applications that are > expecting the tag bits to be cleared. Instead, create a new pair of > union fields in siginfo._sigfault, and store the tag bits of FAR_EL1 > there, together with a mask specifying which bits are valid. > > A flag is added to si_xflags to allow userspace to determine whether > the values in the fields are valid. > > [1] http://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html > > Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx> > Link: https://linux-review.googlesource.com/id/Ia8876bad8c798e0a32df7c2ce1256c4771c81446 > --- > v12: > - add new fields to signal_compat.c test cases > - rebased to 5.10-rc1 > - mask out bits 63:60 for tag check faults > > v11: > - add a comment explaining what the arch hook should do > - rename ignored bits to tag bits > > v10: > - rename the flag to SIXFLAG_ADDR_IGNORED_BITS > - use an arch hook to specify which bits are ignored, instead > of passing them explicitly > - while refactoring for the arch hook, noticed that my previous > patches missed a case involving cache maintenance instructions, > so expose the tag bits for that signal as well > > v9: > - make the ignored bits fields generic > - add some new dependent patches that prepare us to store the > field in such a way that userspace can detect their presence > > v8: > - rebase onto 5.8rc2 > > v7: > - switch to a new siginfo field instead of using sigcontext > - merge the patch back into one since the other patches are now > unnecessary > > v6: > - move fault address and fault code into the kernel_siginfo data structure > - split the patch in three since it was getting large and now has > generic and arch-specific parts > > v5: > - add padding to fault_addr_top_byte_context in order to ensure the correct > size and preserve sp alignment > > v4: > - expose only the tag bits in the context instead of the entire FAR_EL1 > - remove mention of the new context from the sigcontext.__reserved[] note > > v3: > - add documentation to tagged-pointers.rst > - update comments in sigcontext.h > > v2: > - revert changes to hw_breakpoint.c > - rename set_thread_esr to set_thread_far_esr > > Documentation/arm64/tagged-pointers.rst | 21 +++++--- > arch/arm64/include/asm/exception.h | 2 +- > arch/arm64/include/asm/signal.h | 19 +++++++ > arch/arm64/include/asm/system_misc.h | 2 +- > arch/arm64/include/asm/traps.h | 6 +-- > arch/arm64/kernel/debug-monitors.c | 5 +- > arch/arm64/kernel/entry-common.c | 2 - > arch/arm64/kernel/ptrace.c | 7 +-- > arch/arm64/kernel/sys_compat.c | 5 +- > arch/arm64/kernel/traps.c | 29 ++++++----- > arch/arm64/mm/fault.c | 68 ++++++++++++++----------- > arch/x86/kernel/signal_compat.c | 9 +++- > include/linux/compat.h | 2 + > include/linux/signal.h | 16 ++++++ > include/uapi/asm-generic/siginfo.h | 10 ++++ > kernel/signal.c | 18 ++++++- > 16 files changed, 148 insertions(+), 73 deletions(-) > create mode 100644 arch/arm64/include/asm/signal.h > > diff --git a/Documentation/arm64/tagged-pointers.rst b/Documentation/arm64/tagged-pointers.rst > index eab4323609b9..032c09a876f4 100644 > --- a/Documentation/arm64/tagged-pointers.rst > +++ b/Documentation/arm64/tagged-pointers.rst > @@ -53,12 +53,21 @@ visibility. > Preserving tags > --------------- > > -Non-zero tags are not preserved when delivering signals. This means that > -signal handlers in applications making use of tags cannot rely on the > -tag information for user virtual addresses being maintained for fields > -inside siginfo_t. One exception to this rule is for signals raised in > -response to watchpoint debug exceptions, where the tag information will > -be preserved. > +Non-zero tags are not preserved in the fault address fields > +siginfo.si_addr or sigcontext.fault_address when delivering > +signals. This means that signal handlers in applications making use > +of tags cannot rely on the tag information for user virtual addresses > +being maintained in these fields. One exception to this rule is for > +signals raised in response to watchpoint debug exceptions, where the > +tag information will be preserved. > + > +The fault address tag is preserved in the si_addr_tag_bits field > +of siginfo, which is set for signals raised in response to data aborts > +and instruction aborts. The si_addr_tag_bits_mask field indicates > +which bits of the field are valid. The validity of these fields is > +indicated by the SIXFLAG_ADDR_TAG_BITS flag in siginfo.si_xflags, > +and the validity of si_xflags in turn is indicated by the kernel > +indicating support for the sigaction.sa_flags flag SA_XFLAGS. > > The architecture prevents the use of a tagged PC, so the upper byte will > be set to a sign-extension of bit 55 on exception return. > diff --git a/arch/arm64/include/asm/exception.h b/arch/arm64/include/asm/exception.h > index 99b9383cd036..2a8aa1884d8a 100644 > --- a/arch/arm64/include/asm/exception.h > +++ b/arch/arm64/include/asm/exception.h > @@ -32,7 +32,7 @@ static inline u32 disr_to_esr(u64 disr) > } > > asmlinkage void enter_from_user_mode(void); > -void do_mem_abort(unsigned long addr, unsigned int esr, struct pt_regs *regs); > +void do_mem_abort(unsigned long far, unsigned int esr, struct pt_regs *regs); > void do_undefinstr(struct pt_regs *regs); > void do_bti(struct pt_regs *regs); > asmlinkage void bad_mode(struct pt_regs *regs, int reason, unsigned int esr); > diff --git a/arch/arm64/include/asm/signal.h b/arch/arm64/include/asm/signal.h > new file mode 100644 > index 000000000000..46f9b3c61896 > --- /dev/null > +++ b/arch/arm64/include/asm/signal.h > @@ -0,0 +1,19 @@ > +/* SPDX-License-Identifier: GPL-2.0 */ > +#ifndef __ARM64_ASM_SIGNAL_H > +#define __ARM64_ASM_SIGNAL_H > + > +#include <uapi/asm/signal.h> > +#include <uapi/asm/siginfo.h> > + > +static inline unsigned long arch_addr_tag_bits_mask(unsigned long sig, > + unsigned long si_code) > +{ > + if (sig == SIGTRAP && si_code == TRAP_BRKPT) > + return 0; > + if (sig == SIGSEGV && si_code == SEGV_MTESERR) > + return 0xfUL << 56; Should this be 0xffUL << 56? I thought MTE ignored bits 63:60, rather than requiring them to be 0? The whole tag byte may be relevant for debugging purposes, even if only some of the bits are checked in hardware. For consistency, I wonder whether it makes sense to expose the bits as tag bits for watchpoint execeptions, even if they are left un-cleared in si_addr for historical reasons. > + return 0xffUL << 56; > +} > +#define arch_addr_tag_bits_mask arch_addr_tag_bits_mask [...] Cheers ---Dave
