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

From: Russell King - ARM Linux admin <[email-masked]>
Date: Fri, 6 Sep 2019 16:17:59 +0100

On Mon, Sep 02, 2019 at 12:36:56PM -0500, Eric W. Biederman wrote:
> 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.

Sorry, only just got around to this again.  What I came up with is this:

8<===
From: Russell King <[email-masked]>
Subject: [PATCH] ARM: mm: fix alignment

Signed-off-by: Russell King <[email-masked]>
---
 arch/arm/mm/alignment.c | 44 ++++++++++++++++++++++++++++++++++++--------
 1 file changed, 36 insertions(+), 8 deletions(-)

diff --git a/arch/arm/mm/alignment.c b/arch/arm/mm/alignment.c
index 6067fa4de22b..529f54d94709 100644
--- a/arch/arm/mm/alignment.c
+++ b/arch/arm/mm/alignment.c
@@ -765,6 +765,36 @@ do_alignment_t32_to_handler(unsigned long *pinstr, struct pt_regs *regs,
 	return NULL;
 }
 
+static int alignment_get_arm(struct pt_regs *regs, u32 *ip, unsigned long *inst)
+{
+	u32 instr = 0;
+	int fault;
+
+	if (user_mode(regs))
+		fault = get_user(instr, ip);
+	else
+		fault = probe_kernel_address(ip, instr);
+
+	*inst = __mem_to_opcode_arm(instr);
+
+	return fault;
+}
+
+static int alignment_get_thumb(struct pt_regs *regs, u16 *ip, u16 *inst)
+{
+	u16 instr = 0;
+	int fault;
+
+	if (user_mode(regs))
+		fault = get_user(instr, ip);
+	else
+		fault = probe_kernel_address(ip, instr);
+
+	*inst = __mem_to_opcode_thumb16(instr);
+
+	return fault;
+}
+
 static int
 do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 {
@@ -772,10 +802,10 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 	unsigned long instr = 0, instrptr;
 	int (*handler)(unsigned long addr, unsigned long instr, struct pt_regs *regs);
 	unsigned int type;
-	unsigned int fault;
 	u16 tinstr = 0;
 	int isize = 4;
 	int thumb2_32b = 0;
+	int fault;
 
 	if (interrupts_enabled(regs))
 		local_irq_enable();
@@ -784,15 +814,14 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 
 	if (thumb_mode(regs)) {
 		u16 *ptr = (u16 *)(instrptr & ~1);
-		fault = probe_kernel_address(ptr, tinstr);
-		tinstr = __mem_to_opcode_thumb16(tinstr);
+
+		fault = alignment_get_thumb(regs, ptr, &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);
-				tinst2 = __mem_to_opcode_thumb16(tinst2);
+				u16 tinst2;
+				fault = alignment_get_thumb(regs, ptr + 1, &tinst2);
 				instr = __opcode_thumb32_compose(tinstr, tinst2);
 				thumb2_32b = 1;
 			} else {
@@ -801,8 +830,7 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 			}
 		}
 	} else {
-		fault = probe_kernel_address((void *)instrptr, instr);
-		instr = __mem_to_opcode_arm(instr);
+		fault = alignment_get_arm(regs, (void *)instrptr, &instr);
 	}
 
 	if (fault) {
-- 
2.7.4

-- 
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