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Date:   Thu, 13 Oct 2022 11:51:40 +0100
From:   "Russell King (Oracle)" <linux@armlinux.org.uk>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
Cc:     Ard Biesheuvel <ardb@kernel.org>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/2] ARM: Fix some check warnings of tool sparse
Message-ID: <Y0ftvEl3JAkYqkQ1@shell.armlinux.org.uk>
References: <20221010095346.1957-1-thunder.leizhen@huawei.com>
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 <1ab4c651-f0ab-1107-1784-ad255dabc33a@huawei.com>
 <CAMj1kXHn70rRaB=BgCrMoqQxRcq=HzZ0NWFYL+FqV_037PaY3w@mail.gmail.com>
 <c262c991-ad1e-81e6-4909-b8c4a1c036ff@huawei.com>
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Sender: Russell King (Oracle) <linux@armlinux.org.uk>
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On Tue, Oct 11, 2022 at 10:29:58AM +0800, Leizhen (ThunderTown) wrote:
> On 2022/10/10 19:06, Ard Biesheuvel wrote:
> > On Mon, 10 Oct 2022 at 12:58, Leizhen (ThunderTown)
> > <thunder.leizhen@huawei.com> wrote:
> >> On 2022/10/10 18:20, Ard Biesheuvel wrote:
> >>> On Mon, 10 Oct 2022 at 11:56, Zhen Lei <thunder.leizhen@huawei.com> wrote:
> >>>>
> >>>> Fix the following warnings:
> >>>>  warning: incorrect type in initializer (different address spaces)
> >>>>     expected unsigned short [noderef] __user *register __p
> >>>>     got unsigned short [usertype] *
> >>>>  warning: cast removes address space '__user' of expression
> >>>>
> >>>> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> >>>> ---
> >>>>  arch/arm/kernel/traps.c | 10 +++++-----
> >>>>  1 file changed, 5 insertions(+), 5 deletions(-)
> >>>>
> >>>> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
> >>>> index 20b2db6dcd1ced7..34aa80c09c508c1 100644
> >>>> --- a/arch/arm/kernel/traps.c
> >>>> +++ b/arch/arm/kernel/traps.c
> >>>> @@ -188,9 +188,9 @@ static void dump_instr(const char *lvl, struct pt_regs *regs)
> >>>>                         }
> >>>>                 } else {
> >>>>                         if (thumb)
> >>>> -                               bad = get_user(val, &((u16 *)addr)[i]);
> >>>> +                               bad = get_user(val, &((u16 __user *)addr)[i]);
> >>>>                         else
> >>>> -                               bad = get_user(val, &((u32 *)addr)[i]);
> >>>> +                               bad = get_user(val, &((u32 __user *)addr)[i]);
> >>>>                 }
> >>>>
> >>>>                 if (!bad)
> >>>> @@ -455,15 +455,15 @@ asmlinkage void do_undefinstr(struct pt_regs *regs)
> >>>>         if (processor_mode(regs) == SVC_MODE) {
> >>>>  #ifdef CONFIG_THUMB2_KERNEL
> >>>>                 if (thumb_mode(regs)) {
> >>>> -                       instr = __mem_to_opcode_thumb16(((u16 *)pc)[0]);
> >>>> +                       instr = __mem_to_opcode_thumb16(((__force u16 *)pc)[0]);
> >>>
> >>> Shouldn't this be __user as well? (and below)
> >>
> >> unsigned int instr;
> >> void __user *pc;
> >>
> >> The __user can clear the warning, but a new warning will be generated.
> >>
> >> instr = __mem_to_opcode_thumb16(((u16 *)pc)[0]);
> >>       ^new                           ^old
> >>
> >> arch/arm/kernel/traps.c:473:33: warning: dereference of noderef expression
> >>
> > 
> > This is because dereferencing a __user pointer is not permitted.
> > 
> > So this code should be using get_kernel_nofault() here not a plain
> > dereference of PC. So better to fix that properly instead of papering
> > over it with a __force cast just to make sparse happy.
> 
> How about:
> @@ -451,9 +451,9 @@ int call_undef_hook(struct pt_regs *regs, unsigned int instr)
>  asmlinkage void do_undefinstr(struct pt_regs *regs)
>  {
>         unsigned int instr;
> -       void __user *pc;
> +       void *pc;
> 
> -       pc = (void __user *)instruction_pointer(regs);
> +       pc = (void *)instruction_pointer(regs);
> 
>         if (processor_mode(regs) == SVC_MODE) {
>  #ifdef CONFIG_THUMB2_KERNEL
> @@ -497,7 +497,7 @@ asmlinkage void do_undefinstr(struct pt_regs *regs)
>         }
>  #endif
>         arm_notify_die("Oops - undefined instruction", regs,
> -                      SIGILL, ILL_ILLOPC, pc, 0, 6);
> +                      SIGILL, ILL_ILLOPC, (void __user *)pc, 0, 6);
>  }
>  NOKPROBE_SYMBOL(do_undefinstr)
> 
> 
> The 'pc' may come from kernel or user. And I found that all the get_user()
> calls have already done type casts to the pc, except arm_notify_die().
> I think the above changes are reasonable.

If we're going to do that, lets do it properly - I think the above would
need some __force usage to stop sparse complaining, whereas I don't
think this will (untested):

diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
index 3f468ac98592..827cbc022900 100644
--- a/arch/arm/kernel/traps.c
+++ b/arch/arm/kernel/traps.c
@@ -449,36 +449,45 @@ int call_undef_hook(struct pt_regs *regs, unsigned int instr)
 asmlinkage void do_undefinstr(struct pt_regs *regs)
 {
 	unsigned int instr;
-	void __user *pc;
+	unsigned long pc;
 
-	pc = (void __user *)instruction_pointer(regs);
+	pc = instruction_pointer(regs);
 
 	if (processor_mode(regs) == SVC_MODE) {
-#ifdef CONFIG_THUMB2_KERNEL
-		if (thumb_mode(regs)) {
-			instr = __mem_to_opcode_thumb16(((u16 *)pc)[0]);
+		if (IS_ENABLED(CONFIG_THUMB2_KERNEL) && thumb_mode(regs)) {
+			u16 *tpc = (u16 *)pc;
+
+			instr = __mem_to_opcode_thumb16(tpc[0]);
 			if (is_wide_instruction(instr)) {
 				u16 inst2;
-				inst2 = __mem_to_opcode_thumb16(((u16 *)pc)[1]);
+
+				inst2 = __mem_to_opcode_thumb16(tpc[1]);
 				instr = __opcode_thumb32_compose(instr, inst2);
 			}
-		} else
-#endif
-			instr = __mem_to_opcode_arm(*(u32 *) pc);
+		} else {
+			u32 *apc = (u32 *)pc;
+
+			instr = __mem_to_opcode_arm(*apc);
+		}
 	} else if (thumb_mode(regs)) {
-		if (get_user(instr, (u16 __user *)pc))
+		u16 __user *tpc = (u16 __user *)pc;
+
+		if (get_user(instr, tpc))
 			goto die_sig;
 		instr = __mem_to_opcode_thumb16(instr);
 		if (is_wide_instruction(instr)) {
 			unsigned int instr2;
-			if (get_user(instr2, (u16 __user *)pc+1))
+			if (get_user(instr2, tpc + 1))
 				goto die_sig;
 			instr2 = __mem_to_opcode_thumb16(instr2);
 			instr = __opcode_thumb32_compose(instr, instr2);
 		}
 	} else {
-		if (get_user(instr, (u32 __user *)pc))
+		u32 __user *apc = (u32 __user *)pc;
+
+		if (get_user(instr, apc))
 			goto die_sig;
+
 		instr = __mem_to_opcode_arm(instr);
 	}
 
@@ -495,7 +504,7 @@ asmlinkage void do_undefinstr(struct pt_regs *regs)
 	}
 #endif
 	arm_notify_die("Oops - undefined instruction", regs,
-		       SIGILL, ILL_ILLOPC, pc, 0, 6);
+		       SIGILL, ILL_ILLOPC, (void __user *)pc, 0, 6);
 }
 NOKPROBE_SYMBOL(do_undefinstr)
 

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