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Message-ID: <20181127153202.GA27075@linux.intel.com>
Date: Tue, 27 Nov 2018 07:32:03 -0800
From: Sean Christopherson <sean.j.christopherson@...el.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
LKML <linux-kernel@...r.kernel.org>,
Yu-cheng Yu <yu-cheng.yu@...el.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Peter Zijlstra <peterz@...radead.org>,
Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 6/5] x86/fault: Clean up the page fault oops decoder a bit
On Thu, Nov 22, 2018 at 09:41:19AM +0100, Ingo Molnar wrote:
>
> * Andy Lutomirski <luto@...nel.org> wrote:
>
> > One of Linus' favorite hobbies seems to be looking at OOPSes and
> > decoding the error code in his head. This is not one of my favorite
> > hobbies :)
> >
> > Teach the page fault OOPS hander to decode the error code. If it's
> > a !USER fault from user mode, print an explicit note to that effect
> > and print out the addresses of various tables that might cause such
> > an error.
> >
> > With this patch applied, if I intentionally point the LDT at 0x0 and
> > run the x86 selftests, I get:
> >
> > BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
> > HW error: normal kernel read fault
> > This was a system access from user code
> > IDT: 0xfffffe0000000000 (limit=0xfff) GDT: 0xfffffe0000001000 (limit=0x7f)
> > LDTR: 0x50 -- base=0x0 limit=0xfff7
> > TR: 0x40 -- base=0xfffffe0000003000 limit=0x206f
> > PGD 800000000456e067 P4D 800000000456e067 PUD 4623067 PMD 0
> > SMP PTI
> > CPU: 0 PID: 153 Comm: ldt_gdt_64 Not tainted 4.19.0+ #1317
> > Hardware name: ...
> > RIP: 0033:0x401454
>
> I've applied your series, with one small edit, the following message:
>
> > HW error: normal kernel read fault
>
> will IMHO confuse the heck out of users, thinking that their hardware is
> broken...
>
> Yes, the message is accurate, in MM pagefault language it's indeed the HW
> error code, but it's a language very few people speak.
>
> So I edited it over to say '#PF error code'. I also applied a few other
> minor cleanups - see the changelog below.
I responded to the original thread a hair too late...
What about something like this instead of manually handling the case
where error_code==0 so that we get e.g. "[KERNEL] [READ]" instead of
"normal kernel read fault"? Getting "[PROT] [KERNEL] [READ]" seems
useful.
IMO "[normal kernel read fault]" followed by "This was a system access
from user code" is still confusing.
---
8b29ee4351d5c625aa9ca2765f8da5e Mon Sep 17 00:00:00 2001
From: Sean Christopherson <sean.j.christopherson@...el.com>
Date: Tue, 27 Nov 2018 07:09:57 -0800
Subject: [PATCH] x86/fault: Print "KERNEL" and "READ" for #PF error codes
...and explicitly state that it's a "code" that's being printed.
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Rik van Riel <riel@...riel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc: linux-kernel@...r.kernel.org
Cc: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
---
arch/x86/mm/fault.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 2ff25ad33233..510e263c256b 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -660,8 +660,10 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long ad
err_str_append(error_code, err_txt, X86_PF_RSVD, "[RSVD]" );
err_str_append(error_code, err_txt, X86_PF_INSTR, "[INSTR]");
err_str_append(error_code, err_txt, X86_PF_PK, "[PK]" );
-
- pr_alert("#PF error: %s\n", error_code ? err_txt : "[normal kernel read fault]");
+ err_str_append(~error_code, err_txt, X86_PF_USER, "[KERNEL]");
+ err_str_append(~error_code, err_txt, X86_PF_WRITE | X86_PF_INSTR,
+ "[READ]");
+ pr_alert("#PF error code: %s\n", err_txt);
if (!(error_code & X86_PF_USER) && user_mode(regs)) {
struct desc_ptr idt, gdt;
--
2.19.2
>
> Let me know if you have any objections.
>
> Thanks,
>
> Ingo
>
> ===============>
> From a2aa52ab16efbee40ad118ebac4a5e438f5b43ee Mon Sep 17 00:00:00 2001
> From: Ingo Molnar <mingo@...nel.org>
> Date: Thu, 22 Nov 2018 09:34:03 +0100
> Subject: [PATCH] x86/fault: Clean up the page fault oops decoder a bit
>
> - Make the oops messages a bit less scary (don't mention 'HW errors')
>
> - Turn 'PROT USER' (which is visually easily confused with PROT_USER)
> into individual bit descriptors: "[PROT] [USER]".
> This also makes "[normal kernel read fault]" more apparent.
>
> - De-abbreviate variables to make the code easier to read
>
> - Use vertical alignment where appropriate.
>
> - Add comment about string size limits and the helper function.
>
> - Remove unnecessary line breaks.
>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Rik van Riel <riel@...riel.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Yu-cheng Yu <yu-cheng.yu@...el.com>
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> ---
> arch/x86/mm/fault.c | 38 +++++++++++++++++++++++---------------
> 1 file changed, 23 insertions(+), 15 deletions(-)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index f5efbdba2b6d..2ff25ad33233 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -603,10 +603,13 @@ static void show_ldttss(const struct desc_ptr *gdt, const char *name, u16 index)
> name, index, addr, (desc.limit0 | (desc.limit1 << 16)));
> }
>
> -static void errstr(unsigned long ec, char *buf, unsigned long mask,
> - const char *txt)
> +/*
> + * This helper function transforms the #PF error_code bits into
> + * "[PROT] [USER]" type of descriptive, almost human-readable error strings:
> + */
> +static void err_str_append(unsigned long error_code, char *buf, unsigned long mask, const char *txt)
> {
> - if (ec & mask) {
> + if (error_code & mask) {
> if (buf[0])
> strcat(buf, " ");
> strcat(buf, txt);
> @@ -614,10 +617,9 @@ static void errstr(unsigned long ec, char *buf, unsigned long mask,
> }
>
> static void
> -show_fault_oops(struct pt_regs *regs, unsigned long error_code,
> - unsigned long address)
> +show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long address)
> {
> - char errtxt[64];
> + char err_txt[64];
>
> if (!oops_may_print())
> return;
> @@ -646,15 +648,21 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code,
> address < PAGE_SIZE ? "NULL pointer dereference" : "paging request",
> (void *)address);
>
> - errtxt[0] = 0;
> - errstr(error_code, errtxt, X86_PF_PROT, "PROT");
> - errstr(error_code, errtxt, X86_PF_WRITE, "WRITE");
> - errstr(error_code, errtxt, X86_PF_USER, "USER");
> - errstr(error_code, errtxt, X86_PF_RSVD, "RSVD");
> - errstr(error_code, errtxt, X86_PF_INSTR, "INSTR");
> - errstr(error_code, errtxt, X86_PF_PK, "PK");
> - pr_alert("HW error: %s\n", error_code ? errtxt :
> - "normal kernel read fault");
> + err_txt[0] = 0;
> +
> + /*
> + * Note: length of these appended strings including the separation space and the
> + * zero delimiter must fit into err_txt[].
> + */
> + err_str_append(error_code, err_txt, X86_PF_PROT, "[PROT]" );
> + err_str_append(error_code, err_txt, X86_PF_WRITE, "[WRITE]");
> + err_str_append(error_code, err_txt, X86_PF_USER, "[USER]" );
> + err_str_append(error_code, err_txt, X86_PF_RSVD, "[RSVD]" );
> + err_str_append(error_code, err_txt, X86_PF_INSTR, "[INSTR]");
> + err_str_append(error_code, err_txt, X86_PF_PK, "[PK]" );
> +
> + pr_alert("#PF error: %s\n", error_code ? err_txt : "[normal kernel read fault]");
> +
> if (!(error_code & X86_PF_USER) && user_mode(regs)) {
> struct desc_ptr idt, gdt;
> u16 ldtr, tr;
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