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SubjectRe: [PATCH] arm: fix page faults in do_alignment
On Mon, Sep 02, 2019 at 12:36:56PM -0500, Eric W. Biederman wrote:
> Russell King - ARM Linux admin <linux@armlinux.org.uk> writes:
>
> > On Fri, Aug 30, 2019 at 04:02:48PM -0500, Eric W. Biederman wrote:
> >> Russell King - ARM Linux admin <linux@armlinux.org.uk> writes:
> >>
> >> > On Fri, Aug 30, 2019 at 02:45:36PM -0500, Eric W. Biederman wrote:
> >> >> Russell King - ARM Linux admin <linux@armlinux.org.uk> 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 <jingxiangfeng@huawei.com>
> >> >> >
> >> >> > 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 <rmk+kernel@armlinux.org.uk>
Subject: [PATCH] ARM: mm: fix alignment

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
---
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
--
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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