[PATCH v3 12/25] arm64: handle PKEY/POE faults

[Joey Gouly <joey.gouly@xxxxxxx>]

If a memory fault occurs that is due to an overlay/pkey fault, report that to
userspace with a SEGV_PKUERR.

Signed-off-by: Joey Gouly <joey.gouly@xxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
---
 arch/arm64/include/asm/traps.h |  1 +
 arch/arm64/kernel/traps.c      | 12 ++++++++--
 arch/arm64/mm/fault.c          | 44 +++++++++++++++++++++++++++++++---
 3 files changed, 52 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/traps.h b/arch/arm64/include/asm/traps.h
index eefe766d6161..f6f6f2cb7f10 100644
--- a/arch/arm64/include/asm/traps.h
+++ b/arch/arm64/include/asm/traps.h
@@ -25,6 +25,7 @@ try_emulate_armv8_deprecated(struct pt_regs *regs, u32 insn)
 void force_signal_inject(int signal, int code, unsigned long address, unsigned long err);
 void arm64_notify_segfault(unsigned long addr);
 void arm64_force_sig_fault(int signo, int code, unsigned long far, const char *str);
+void arm64_force_sig_fault_pkey(int signo, int code, unsigned long far, const char *str, int pkey);
 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);
 
diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index 215e6d7f2df8..1bac6c84d3f5 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -263,16 +263,24 @@ static void arm64_show_signal(int signo, const char *str)
 	__show_regs(regs);
 }
 
-void arm64_force_sig_fault(int signo, int code, unsigned long far,
-			   const char *str)
+void arm64_force_sig_fault_pkey(int signo, int code, unsigned long far,
+			   const char *str, int pkey)
 {
 	arm64_show_signal(signo, str);
 	if (signo == SIGKILL)
 		force_sig(SIGKILL);
+	else if (code == SEGV_PKUERR)
+		force_sig_pkuerr((void __user *)far, pkey);
 	else
 		force_sig_fault(signo, code, (void __user *)far);
 }
 
+void arm64_force_sig_fault(int signo, int code, unsigned long far,
+			   const char *str)
+{
+	arm64_force_sig_fault_pkey(signo, code, far, str, 0);
+}
+
 void arm64_force_sig_mceerr(int code, unsigned long far, short lsb,
 			    const char *str)
 {
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 460d799e1296..efd263f56da7 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -23,6 +23,7 @@
 #include <linux/sched/debug.h>
 #include <linux/highmem.h>
 #include <linux/perf_event.h>
+#include <linux/pkeys.h>
 #include <linux/preempt.h>
 #include <linux/hugetlb.h>
 
@@ -497,6 +498,23 @@ static void do_bad_area(unsigned long far, unsigned long esr,
 #define VM_FAULT_BADMAP		((__force vm_fault_t)0x010000)
 #define VM_FAULT_BADACCESS	((__force vm_fault_t)0x020000)
 
+static bool fault_from_pkey(unsigned long esr, struct vm_area_struct *vma,
+			unsigned int mm_flags)
+{
+	unsigned long iss2 = ESR_ELx_ISS2(esr);
+
+	if (!arch_pkeys_enabled())
+		return false;
+
+	if (iss2 & ESR_ELx_Overlay)
+		return true;
+
+	return !arch_vma_access_permitted(vma,
+			mm_flags & FAULT_FLAG_WRITE,
+			mm_flags & FAULT_FLAG_INSTRUCTION,
+			mm_flags & FAULT_FLAG_REMOTE);
+}
+
 static vm_fault_t __do_page_fault(struct mm_struct *mm,
 				  struct vm_area_struct *vma, unsigned long addr,
 				  unsigned int mm_flags, unsigned long vm_flags,
@@ -688,9 +706,29 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr,
 		 * Something tried to access memory that isn't in our memory
 		 * map.
 		 */
-		arm64_force_sig_fault(SIGSEGV,
-				      fault == VM_FAULT_BADACCESS ? SEGV_ACCERR : SEGV_MAPERR,
-				      far, inf->name);
+		int fault_kind;
+		/*
+		 * The pkey value that we return to userspace can be different
+		 * from the pkey that caused the fault.
+		 *
+		 * 1. T1   : mprotect_key(foo, PAGE_SIZE, pkey=4);
+		 * 2. T1   : set AMR to deny access to pkey=4, touches, page
+		 * 3. T1   : faults...
+		 * 4.    T2: mprotect_key(foo, PAGE_SIZE, pkey=5);
+		 * 5. T1   : enters fault handler, takes mmap_lock, etc...
+		 * 6. T1   : reaches here, sees vma_pkey(vma)=5, when we really
+		 *	     faulted on a pte with its pkey=4.
+		 */
+		int pkey = vma_pkey(vma);
+
+		if (fault_from_pkey(esr, vma, mm_flags))
+			fault_kind = SEGV_PKUERR;
+		else
+			fault_kind = fault == VM_FAULT_BADACCESS ? SEGV_ACCERR : SEGV_MAPERR;
+
+		arm64_force_sig_fault_pkey(SIGSEGV,
+				      fault_kind,
+				      far, inf->name, pkey);
 	}
 
 	return 0;
-- 
2.25.1