Subject: Re: [PATCH] arm: fix page faults in do_alignment

From: [email-masked] (Eric W. Biederman)
Date: Mon, 02 Sep 2019 12:36:56 -0500

Russell King - ARM Linux admin <[email-masked]> writes:

> On Fri, Aug 30, 2019 at 04:02:48PM -0500, Eric W. Biederman wrote:
>> Russell King - ARM Linux admin <[email-masked]> writes:
>> 
>> > On Fri, Aug 30, 2019 at 02:45:36PM -0500, Eric W. Biederman wrote:
>> >> Russell King - ARM Linux admin <[email-masked]> writes:
>> >> 
>> >> > On Fri, Aug 30, 2019 at 09:31:17PM +0800, Jing Xiangfeng wrote:
>> >> >> The function do_alignment can handle misaligned address for user and
>> >> >> kernel space. If it is a userspace access, do_alignment may fail on
>> >> >> a low-memory situation, because page faults are disabled in
>> >> >> probe_kernel_address.
>> >> >> 
>> >> >> Fix this by using __copy_from_user stead of probe_kernel_address.
>> >> >> 
>> >> >> Fixes: b255188 ("ARM: fix scheduling while atomic warning in alignment handling code")
>> >> >> Signed-off-by: Jing Xiangfeng <[email-masked]>
>> >> >
>> >> > NAK.
>> >> >
>> >> > The "scheduling while atomic warning in alignment handling code" is
>> >> > caused by fixing up the page fault while trying to handle the
>> >> > mis-alignment fault generated from an instruction in atomic context.
>> >> >
>> >> > Your patch re-introduces that bug.
>> >> 
>> >> And the patch that fixed scheduling while atomic apparently introduced a
>> >> regression.  Admittedly a regression that took 6 years to track down but
>> >> still.
>> >
>> > Right, and given the number of years, we are trading one regression for
>> > a different regression.  If we revert to the original code where we
>> > fix up, we will end up with people complaining about a "new" regression
>> > caused by reverting the previous fix.  Follow this policy and we just
>> > end up constantly reverting the previous revert.
>> >
>> > The window is very small - the page in question will have had to have
>> > instructions read from it immediately prior to the handler being entered,
>> > and would have had to be made "old" before subsequently being unmapped.
>> 
>> > Rather than excessively complicating the code and making it even more
>> > inefficient (as in your patch), we could instead retry executing the
>> > instruction when we discover that the page is unavailable, which should
>> > cause the page to be paged back in.
>> 
>> My patch does not introduce any inefficiencies.  It onlys moves the
>> check for user_mode up a bit.  My patch did duplicate the code.
>> 
>> > If the page really is unavailable, the prefetch abort should cause a
>> > SEGV to be raised, otherwise the re-execution should replace the page.
>> >
>> > The danger to that approach is we page it back in, and it gets paged
>> > back out before we're able to read the instruction indefinitely.
>> 
>> I would think either a little code duplication or a function that looks
>> at user_mode(regs) and picks the appropriate kind of copy to do would be
>> the best way to go.  Because what needs to happen in the two cases for
>> reading the instruction are almost completely different.
>
> That is what I mean.  I'd prefer to avoid that with the large chunk of
> code.  How about instead adding a local replacement for
> probe_kernel_address() that just sorts out the reading, rather than
> duplicating all the code to deal with thumb fixup.

So something like this should be fine?

Jing Xiangfeng can you test this please?  I think this fixes your issue
but I don't currently have an arm development box where I could test this.

diff --git a/arch/arm/mm/alignment.c b/arch/arm/mm/alignment.c
index 04b36436cbc0..b07d17ca0ae5 100644
--- a/arch/arm/mm/alignment.c
+++ b/arch/arm/mm/alignment.c
@@ -767,6 +767,23 @@ do_alignment_t32_to_handler(unsigned long *pinstr, struct pt_regs *regs,
 	return NULL;
 }
 
+static inline unsigned long
+copy_instr(bool umode, void *dst, unsigned long instrptr, size_t size)
+{
+	unsigned long result;
+	if (umode) {
+		void __user *src = (void *)instrptr;
+		result = copy_from_user(dst, src, size);
+	} else {
+		void *src = (void *)instrptr;
+		result = probe_kernel_read(dst, src, size);
+	}
+	/* Convert short reads into -EFAULT */
+	if ((result >= 0) && (result < size))
+		result = -EFAULT;
+	return result;
+}
+
 static int
 do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 {
@@ -778,22 +795,24 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 	u16 tinstr = 0;
 	int isize = 4;
 	int thumb2_32b = 0;
+	bool umode;
 
 	if (interrupts_enabled(regs))
 		local_irq_enable();
 
 	instrptr = instruction_pointer(regs);
+	umode = user_mode(regs);
 
 	if (thumb_mode(regs)) {
-		u16 *ptr = (u16 *)(instrptr & ~1);
-		fault = probe_kernel_address(ptr, tinstr);
+		unsigned long tinstrptr = instrptr & ~1;
+		fault = copy_instr(umode, &tinstr, tinstrptr, 2);
 		tinstr = __mem_to_opcode_thumb16(tinstr);
 		if (!fault) {
 			if (cpu_architecture() >= CPU_ARCH_ARMv7 &&
 			    IS_T32(tinstr)) {
 				/* Thumb-2 32-bit */
 				u16 tinst2 = 0;
-				fault = probe_kernel_address(ptr + 1, tinst2);
+				fault = copy_instr(umode, &tinst2, tinstrptr + 2, 2);
 				tinst2 = __mem_to_opcode_thumb16(tinst2);
 				instr = __opcode_thumb32_compose(tinstr, tinst2);
 				thumb2_32b = 1;
@@ -803,7 +822,7 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 			}
 		}
 	} else {
-		fault = probe_kernel_address((void *)instrptr, instr);
+		fault = copy_instr(umode, &instr, instrptr, 4);
 		instr = __mem_to_opcode_arm(instr);
 	}
 
@@ -812,7 +831,7 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 		goto bad_or_fault;
 	}
 
-	if (user_mode(regs))
+	if (umode)
 		goto user;
 
 	ai_sys += 1;

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