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Subject[tip:x86/mm] x86/fault: Decode and print #PF oops in human readable form
Commit-ID:  18ea35c5ed9921867194a8efc2a0ac2d5a3c7d2a
Gitweb: https://git.kernel.org/tip/18ea35c5ed9921867194a8efc2a0ac2d5a3c7d2a
Author: Sean Christopherson <sean.j.christopherson@intel.com>
AuthorDate: Fri, 21 Dec 2018 13:36:57 -0800
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 19 Apr 2019 19:31:16 +0200

x86/fault: Decode and print #PF oops in human readable form

Linus pointed out that deciphering the raw #PF error code and printing
a more human readable message are two different things, and also that
printing the negative cases is mostly just noise[1]. For example, the
USER bit doesn't mean the fault originated in user code and stating
that an oops wasn't due to a protection keys violation isn't interesting
since an oops on a keys violation is a one-in-a-million scenario.

Remove the per-bit decoding of the error code and instead print:
- the raw error code
- why the fault occurred
- the effective privilege level of the access
- the type of access
- whether the fault originated in user code or kernel code

This provides the user with the information needed to triage 99.9% of
oopses without polluting the log with useless information or conflating
the error_code with the CPL.

Sample output:

BUG: kernel NULL pointer dereference, address = 0000000000000008
#PF: supervisor-privileged instruction fetch from kernel code
#PF: error_code(0x0010) - not-present page

BUG: unable to handle page fault for address = ffffbeef00000000
#PF: supervisor-privileged instruction fetch from kernel code
#PF: error_code(0x0010) - not-present page

BUG: unable to handle page fault for address = ffffc90000230000
#PF: supervisor-privileged write access from kernel code
#PF: error_code(0x000b) - reserved bit violation

[1] https://lkml.kernel.org/r/CAHk-=whk_fsnxVMvF1T2fFCaP2WrvSybABrLQCWLJyCvHw6NKA@mail.gmail.com

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>
Link: http://lkml.kernel.org/r/20181221213657.27628-3-sean.j.christopherson@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/mm/fault.c | 42 +++++++++++-------------------------------
1 file changed, 11 insertions(+), 31 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index df2c5cdef5c4..74c9204c5751 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -603,24 +603,9 @@ static void show_ldttss(const struct desc_ptr *gdt, const char *name, u16 index)
name, index, addr, (desc.limit0 | (desc.limit1 << 16)));
}

-/*
- * 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 (error_code & mask) {
- if (buf[0])
- strcat(buf, " ");
- strcat(buf, txt);
- }
-}
-
static void
show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long address)
{
- char err_txt[64];
-
if (!oops_may_print())
return;

@@ -651,27 +636,22 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long ad
pr_alert("BUG: unable to handle page fault for address = %px\n",
(void *)address);

- 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]");
+ pr_alert("#PF: %s-privileged %s from %s code\n",
+ (error_code & X86_PF_USER) ? "user" : "supervisor",
+ (error_code & X86_PF_INSTR) ? "instruction fetch" :
+ (error_code & X86_PF_WRITE) ? "write access" :
+ "read access",
+ user_mode(regs) ? "user" : "kernel");
+ pr_alert("#PF: error_code(0x%04lx) - %s\n", error_code,
+ !(error_code & X86_PF_PROT) ? "not-present page" :
+ (error_code & X86_PF_RSVD) ? "reserved bit violation" :
+ (error_code & X86_PF_PK) ? "protection keys violation" :
+ "permissions violation");

if (!(error_code & X86_PF_USER) && user_mode(regs)) {
struct desc_ptr idt, gdt;
u16 ldtr, tr;

- pr_alert("This was a system access from user code\n");
-
/*
* This can happen for quite a few reasons. The more obvious
* ones are faults accessing the GDT, or LDT. Perhaps
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