Commit dceec3ff authored by Peter Collingbourne's avatar Peter Collingbourne Committed by Catalin Marinas
Browse files

arm64: expose FAR_EL1 tag bits in siginfo 

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).

Expose these bits via the arch_untagged_si_addr mechanism, so that
they are only exposed to signal handlers with the SA_EXPOSE_TAGBITS
flag set.

[1] http://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html



Signed-off-by: default avatarPeter Collingbourne <pcc@google.com>
Reviewed-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
Link: https://linux-review.googlesource.com/id/Ia8876bad8c798e0a32df7c2ce1256c4771c81446
Link: https://lore.kernel.org/r/0010296597784267472fa13b39f8238d87a72cf8.1605904350.git.pcc@google.com


Signed-off-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
parent 6ac05e83

Documentation/arm64/tagged-pointers.rst

+19 −6
Original line number Diff line number Diff line
@@ -53,12 +53,25 @@ 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.

When delivering signals, non-zero tags are not preserved in

siginfo.si_addr unless the flag SA_EXPOSE_TAGBITS was set in

sigaction.sa_flags when the signal handler was installed. 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 unless the flag was set.



Due to architecture limitations, bits 63:60 of the fault address

are not preserved in response to synchronous tag check faults

(SEGV_MTESERR) even if SA_EXPOSE_TAGBITS was set. Applications should

treat the values of these bits as undefined in order to accommodate

future architecture revisions which may preserve the bits.



For signals raised in response to watchpoint debug exceptions, the

tag information will be preserved regardless of the SA_EXPOSE_TAGBITS

flag setting.



Non-zero tags are never preserved in sigcontext.fault_address

regardless of the SA_EXPOSE_TAGBITS flag setting.



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.

arch/arm64/include/asm/exception.h

+1 −1
Original line number Diff line number Diff line
@@ -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);

arch/arm64/include/asm/signal.h

0 → 100644
+25 −0
Original line number Diff line number Diff line 
/* SPDX-License-Identifier: GPL-2.0 */

#ifndef __ARM64_ASM_SIGNAL_H

#define __ARM64_ASM_SIGNAL_H



#include <asm/memory.h>

#include <uapi/asm/signal.h>

#include <uapi/asm/siginfo.h>



static inline void __user *arch_untagged_si_addr(void __user *addr,

						 unsigned long sig,

						 unsigned long si_code)

{

	/*

	 * For historical reasons, all bits of the fault address are exposed as

	 * address bits for watchpoint exceptions. New architectures should

	 * handle the tag bits consistently.

	 */

	if (sig == SIGTRAP && si_code == TRAP_BRKPT)

		return addr;



	return untagged_addr(addr);

}

#define arch_untagged_si_addr arch_untagged_si_addr



#endif

arch/arm64/include/asm/system_misc.h

+1 −1
Original line number Diff line number Diff line
@@ -22,7 +22,7 @@ void die(const char *msg, struct pt_regs *regs, int err); 


struct siginfo;

void arm64_notify_die(const char *str, struct pt_regs *regs,

		      int signo, int sicode, void __user *addr,

		      int signo, int sicode, unsigned long far,

		      int err);



void hook_debug_fault_code(int nr, int (*fn)(unsigned long, unsigned int,

arch/arm64/include/asm/traps.h

+3 −3
Original line number Diff line number Diff line
@@ -26,9 +26,9 @@ void register_undef_hook(struct undef_hook *hook); 
void unregister_undef_hook(struct undef_hook *hook);

void force_signal_inject(int signal, int code, unsigned long address, unsigned int err);

void arm64_notify_segfault(unsigned long addr);

void arm64_force_sig_fault(int signo, int code, void __user *addr, const char *str);

void arm64_force_sig_mceerr(int code, void __user *addr, short lsb, const char *str);

void arm64_force_sig_ptrace_errno_trap(int errno, void __user *addr, const char *str);

void arm64_force_sig_fault(int signo, int code, unsigned long far, const char *str);

void arm64_force_sig_mceerr(int code, unsigned long far, short lsb, const char *str);

void arm64_force_sig_ptrace_errno_trap(int errno, unsigned long far, const char *str);



/*

 * Move regs->pc to next instruction and do necessary setup before it