powerpc/book3s64/kuap: Improve error reporting with KUAP

This partially reverts commit eb232b1 ("powerpc/book3s64/kuap: Improve
error reporting with KUAP") and update the fault handler to print

[   55.022514] Kernel attempted to access user page (7e6725b70000) - exploit attempt? (uid: 0)
[   55.022528] BUG: Unable to handle kernel data access on read at 0x7e6725b70000
[   55.022533] Faulting instruction address: 0xc000000000e8b9bc
[   55.022540] Oops: Kernel access of bad area, sig: 11 [#1]
....

when the kernel access userspace address without unlocking AMR.

bad_kuap_fault() is added as part of commit 5e5be3a ("powerpc/mm: Detect
bad KUAP faults") to catch userspace access incorrectly blocked by AMR. Hence
retain the full stack dump there even with hash translation. Also, add a comment
explaining the difference between hash and radix.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201208031539.84878-1-aneesh.kumar@linux.ibm.com

arch/powerpc/include/asm/book3s/32/kup.h

@@ -177,8 +177,8 @@ static inline void restore_user_access(unsigned long flags)
 		allow_user_access(to, to, end - addr, KUAP_READ_WRITE);
 }
 
-static inline bool bad_kuap_fault(struct pt_regs *regs, unsigned long address,
-				  bool is_write, unsigned long error_code)
+static inline bool
+bad_kuap_fault(struct pt_regs *regs, unsigned long address, bool is_write)
 {
 	unsigned long begin = regs->kuap & 0xf0000000;
 	unsigned long end = regs->kuap << 28;

arch/powerpc/include/asm/book3s/64/kup.h

@@ -353,29 +353,29 @@ static inline void set_kuap(unsigned long value)
 	isync();
 }
 
-#define RADIX_KUAP_BLOCK_READ	UL(0x4000000000000000)
-#define RADIX_KUAP_BLOCK_WRITE	UL(0x8000000000000000)
-
 static inline bool bad_kuap_fault(struct pt_regs *regs, unsigned long address,
-				  bool is_write, unsigned long error_code)
+				  bool is_write)
 {
 	if (!mmu_has_feature(MMU_FTR_BOOK3S_KUAP))
 		return false;
-
-	if (radix_enabled()) {
-		/*
-		 * Will be a storage protection fault.
-		 * Only check the details of AMR[0]
-		 */
-		return WARN((regs->kuap & (is_write ? RADIX_KUAP_BLOCK_WRITE : RADIX_KUAP_BLOCK_READ)),
-			    "Bug: %s fault blocked by AMR!", is_write ? "Write" : "Read");
-	}
 	/*
-	 * We don't want to WARN here because userspace can setup
-	 * keys such that a kernel access to user address can cause
-	 * fault
+	 * For radix this will be a storage protection fault (DSISR_PROTFAULT).
+	 * For hash this will be a key fault (DSISR_KEYFAULT)
 	 */
-	return !!(error_code & DSISR_KEYFAULT);
+	/*
+	 * We do have exception table entry, but accessing the
+	 * userspace results in fault.  This could be because we
+	 * didn't unlock the AMR or access is denied by userspace
+	 * using a key value that blocks access. We are only interested
+	 * in catching the use case of accessing without unlocking
+	 * the AMR. Hence check for BLOCK_WRITE/READ against AMR.
+	 */
+	if (is_write) {
+		return WARN(((regs->amr & AMR_KUAP_BLOCK_WRITE) == AMR_KUAP_BLOCK_WRITE),
+			    "Bug: Write fault blocked by AMR!");
+	}
+	return WARN(((regs->amr & AMR_KUAP_BLOCK_READ) == AMR_KUAP_BLOCK_READ),
+		    "Bug: Read fault blocked by AMR!");
 }
 
 static __always_inline void allow_user_access(void __user *to, const void __user *from,

arch/powerpc/include/asm/kup.h

@@ -62,8 +62,8 @@ void setup_kuap(bool disabled);
 #else
 static inline void setup_kuap(bool disabled) { }
 
-static inline bool bad_kuap_fault(struct pt_regs *regs, unsigned long address,
-				  bool is_write, unsigned long error_code)
+static inline bool
+bad_kuap_fault(struct pt_regs *regs, unsigned long address, bool is_write)
 {
 	return false;
 }

arch/powerpc/include/asm/nohash/32/kup-8xx.h

@@ -60,8 +60,8 @@ static inline void restore_user_access(unsigned long flags)
 	mtspr(SPRN_MD_AP, flags);
 }
 
-static inline bool bad_kuap_fault(struct pt_regs *regs, unsigned long address,
-				  bool is_write, unsigned long error_code)
+static inline bool
+bad_kuap_fault(struct pt_regs *regs, unsigned long address, bool is_write)
 {
 	return WARN(!((regs->kuap ^ MD_APG_KUAP) & 0xff000000),
 		    "Bug: fault blocked by AP register !");

arch/powerpc/mm/fault.c

@@ -210,7 +210,7 @@ static bool bad_kernel_fault(struct pt_regs *regs, unsigned long error_code,
 		return true;
 	}
 
-	if (!is_exec && address < TASK_SIZE && (error_code & DSISR_PROTFAULT) &&
+	if (!is_exec && address < TASK_SIZE && (error_code & (DSISR_PROTFAULT | DSISR_KEYFAULT)) &&
 	    !search_exception_tables(regs->nip)) {
 		pr_crit_ratelimited("Kernel attempted to access user page (%lx) - exploit attempt? (uid: %d)\n",
 				    address,
@@ -227,7 +227,7 @@ static bool bad_kernel_fault(struct pt_regs *regs, unsigned long error_code,
 
 	// Read/write fault in a valid region (the exception table search passed
 	// above), but blocked by KUAP is bad, it can never succeed.
-	if (bad_kuap_fault(regs, address, is_write, error_code))
+	if (bad_kuap_fault(regs, address, is_write))
 		return true;
 
 	// What's left? Kernel fault on user in well defined regions (extable