Patch "arm64: fix dump_instr when PAN and UAO are in use" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

arm64: fix dump_instr when PAN and UAO are in use

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
arm64-fix-dump_instr-when-pan-and-uao-are-in-use.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Mark Rutland <[email protected]>
Date: Mon, 13 Jun 2016 11:15:14 +0100
Subject: arm64: fix dump_instr when PAN and UAO are in use

From: Mark Rutland <[email protected]>

commit c5cea06be060f38e5400d796e61cfc8c36e52924 upstream.

If the kernel is set to show unhandled signals, and a user task does not handle a SIGILL as a result of an instruction abort, we will attempt to log the offending instruction with dump_instr before killing the task.

We use dump_instr to log the encoding of the offending userspace instruction. However, dump_instr is also used to dump instructions from kernel space, and internally always switches to KERNEL_DS before dumping the instruction with get_user. When both PAN and UAO are in use, reading a user instruction via get_user while in KERNEL_DS will result in a permission fault, which leads to an Oops.

As we have regs corresponding to the context of the original instruction abort, we can inspect this and only flip to KERNEL_DS if the original abort was taken from the kernel, avoiding this issue. At the same time, remove the redundant (and incorrect) comments regarding the order dump_mem and dump_instr are called in.

---
 arch/arm64/kernel/traps.c |   26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

@@ -64,8 +64,7 @@ static void dump_mem(const char *lvl, co
 
 	/*
 	 * We need to switch to kernel mode so that we can use __get_user
-	 * to safely read from kernel space.  Note that we now dump the
-	 * code first, just in case the backtrace kills us.
+	 * to safely read from kernel space.
 	 */
 	fs = get_fs();
 	set_fs(KERNEL_DS);

@@ -111,21 +110,12 @@ static void dump_backtrace_entry(unsigne
 	print_ip_sym(where);
 }
 
-static void dump_instr(const char *lvl, struct pt_regs *regs)
+static void __dump_instr(const char *lvl, struct pt_regs *regs)
 {
 	unsigned long addr = instruction_pointer(regs);
-	mm_segment_t fs;
 	char str[sizeof("00000000 ") * 5 + 2 + 1], *p = str;
 	int i;
 
-	/*
-	 * We need to switch to kernel mode so that we can use __get_user
-	 * to safely read from kernel space.  Note that we now dump the
-	 * code first, just in case the backtrace kills us.
-	 */
-	fs = get_fs();
-	set_fs(KERNEL_DS);
-
 	for (i = -4; i < 1; i++) {
 		unsigned int val, bad;
 

@@ -139,8 +129,18 @@ static void dump_instr(const char *lvl,
 		}
 	}
 	printk("%sCode: %s\n", lvl, str);
+}
 
-	set_fs(fs);
+static void dump_instr(const char *lvl, struct pt_regs *regs)
+{
+	if (!user_mode(regs)) {
+		mm_segment_t fs = get_fs();
+		set_fs(KERNEL_DS);
+		__dump_instr(lvl, regs);
+		set_fs(fs);
+	} else {
+		__dump_instr(lvl, regs);
+	}
 }
 
 static void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/arm64-fix-dump_instr-when-pan-and-uao-are-in-use.patch