From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: x86@kernel.org, LKML <linux-kernel@vger.kernel.org>,
Yu-cheng Yu <yu-cheng.yu@intel.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
Peter Zijlstra <peterz@infradead.org>,
Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH 12/13] x86/fault: Decode page fault OOPSes better
Date: Tue, 27 Nov 2018 06:46:04 -0800 [thread overview]
Message-ID: <20181127144603.GA3130@linux.intel.com> (raw)
In-Reply-To: <9c621efcf6a86f7d215941dab3dca0acd1274638.1542667307.git.luto@kernel.org>
On Mon, Nov 19, 2018 at 02:45:36PM -0800, Andy Lutomirski 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
>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
> arch/x86/mm/fault.c | 84 +++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 84 insertions(+)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 092ed6b1df8a..f34241fcc633 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -27,6 +27,7 @@
> #include <asm/vm86.h> /* struct vm86 */
> #include <asm/mmu_context.h> /* vma_pkey() */
> #include <asm/efi.h> /* efi_recover_from_page_fault()*/
> +#include <asm/desc.h> /* store_idt(), ... */
>
> #define CREATE_TRACE_POINTS
> #include <asm/trace/exceptions.h>
> @@ -571,10 +572,53 @@ static int is_f00f_bug(struct pt_regs *regs, unsigned long address)
> return 0;
> }
>
> +static void show_ldttss(const struct desc_ptr *gdt, const char *name, u16 index)
> +{
> + u32 offset = (index >> 3) * sizeof(struct desc_struct);
> + unsigned long addr;
> + struct ldttss_desc desc;
> +
> + if (index == 0) {
> + pr_alert("%s: NULL\n", name);
> + return;
> + }
> +
> + if (offset + sizeof(struct ldttss_desc) >= gdt->size) {
> + pr_alert("%s: 0x%hx -- out of bounds\n", name, index);
> + return;
> + }
> +
> + if (probe_kernel_read(&desc, (void *)(gdt->address + offset),
> + sizeof(struct ldttss_desc))) {
> + pr_alert("%s: 0x%hx -- GDT entry is not readable\n",
> + name, index);
> + return;
> + }
> +
> + addr = desc.base0 | (desc.base1 << 16) | (desc.base2 << 24);
> +#ifdef CONFIG_X86_64
> + addr |= ((u64)desc.base3 << 32);
> +#endif
> + pr_alert("%s: 0x%hx -- base=0x%lx limit=0x%x\n",
> + name, index, addr, (desc.limit0 | (desc.limit1 << 16)));
> +}
> +
> +static void errstr(unsigned long ec, char *buf, unsigned long mask,
> + const char *txt)
> +{
> + if (ec & 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 errtxt[64];
> +
> if (!oops_may_print())
> return;
>
> @@ -602,6 +646,46 @@ 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");
What about something like this instead of manually handling the case
where error_code==0 so that we get e.g. "!PROT KERNEL READ" instead of
"normal kernel read fault"? Not sure !PROT and/or KERNEL are needed,
but getting at least "PROT READ" seems useful.
errstr(!error_code, errtxt, X86_PF_PROT, "!PROT");
errstr(!error_code, errtxt, X86_PF_USER, "KERNEL");
errstr(!error_code, errtxt, X86_PF_WRITE | X86_PF_INSTR, "READ");
And change the pr_alert to "HW error code:"?
The original is confusing (to me) because "HW error: normal kernel read fault"
obfuscates the fact that we're printing the #PF error code, i.e. it looks
like an arbitrary kernel message.
This:
HW error code: !PROT KERNEL READ
This was a system access from user code
or:
HW error code: !PROT READ
This was a system access from user code
or:
HW error code: KERNEL READ
This was a system access from user code
or:
HW error code: READ
This was a system access from user code
are all less confusing IMO.
> + 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
> + * surprisingly, if the CPU tries to deliver a benign or
> + * contributory exception from user code and gets a page fault
> + * during delivery, the page fault can be delivered as though
> + * it originated directly from user code. This could happen
> + * due to wrong permissions on the IDT, GDT, LDT, TSS, or
> + * kernel or IST stack.
> + */
> + store_idt(&idt);
> +
> + /* Usable even on Xen PV -- it's just slow. */
> + native_store_gdt(&gdt);
> +
> + pr_alert("IDT: 0x%lx (limit=0x%hx) GDT: 0x%lx (limit=0x%hx)\n",
> + idt.address, idt.size, gdt.address, gdt.size);
> +
> + store_ldt(ldtr);
> + show_ldttss(&gdt, "LDTR", ldtr);
> +
> + store_tr(tr);
> + show_ldttss(&gdt, "TR", tr);
> + }
> +
> dump_pagetable(address);
> }
>
> --
> 2.17.2
>
next prev parent reply other threads:[~2018-11-27 14:46 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-19 22:45 [PATCH 00/13] x86/fault: #PF improvements, mostly related to USER bit Andy Lutomirski
2018-11-19 22:45 ` [PATCH 01/13] x86/fault: Check user_mode(regs) when avoiding an mmap_sem deadlock Andy Lutomirski
2018-11-20 8:14 ` [tip:x86/mm] " tip-bot for Andy Lutomirski
2018-11-20 8:15 ` [PATCH 01/13] " Peter Zijlstra
2018-11-19 22:45 ` [PATCH 02/13] x86/fault: Check user_mode(regs) when validating a stack extension Andy Lutomirski
2018-11-20 7:39 ` Ingo Molnar
2018-11-20 8:13 ` Ingo Molnar
2018-11-19 22:45 ` [PATCH 03/13] x86/cpufeatures, x86/fault: Mark SMAP as disabled when configured out Andy Lutomirski
2018-11-20 8:15 ` [tip:x86/mm] " tip-bot for Andy Lutomirski
2018-11-19 22:45 ` [PATCH 04/13] x86/fault: Fold smap_violation() into do_user_addr_fault() Andy Lutomirski
2018-11-20 8:15 ` [tip:x86/mm] " tip-bot for Andy Lutomirski
2018-11-19 22:45 ` [PATCH 05/13] x86/fault: Fix SMAP #PF handling buglet for implicit supervisor accesses Andy Lutomirski
2018-11-20 8:16 ` [tip:x86/mm] " tip-bot for Andy Lutomirski
2018-11-19 22:45 ` [PATCH 06/13] x86/fault: Improve the condition for signalling vs OOPSing Andy Lutomirski
2018-11-20 8:16 ` [tip:x86/mm] " tip-bot for Andy Lutomirski
2018-11-19 22:45 ` [PATCH 07/13] x86/fault: Make error_code sanitization more robust Andy Lutomirski
2018-11-20 8:17 ` [tip:x86/mm] " tip-bot for Andy Lutomirski
2018-11-19 22:45 ` [PATCH 08/13] x86/fault: Don't set thread.cr2, etc before OOPSing Andy Lutomirski
2018-11-20 8:17 ` [tip:x86/mm] " tip-bot for Andy Lutomirski
2018-11-19 22:45 ` [PATCH 09/13] x86/fault: Remove sw_error_code Andy Lutomirski
2018-11-19 22:45 ` [PATCH 10/13] x86/fault: Don't try to recover from an implicit supervisor access Andy Lutomirski
2018-11-19 22:45 ` [PATCH 11/13] x86/oops: Show the correct CS value in show_regs() Andy Lutomirski
2018-11-19 22:45 ` [PATCH 12/13] x86/fault: Decode page fault OOPSes better Andy Lutomirski
2018-11-27 14:46 ` Sean Christopherson [this message]
2018-11-19 22:45 ` [PATCH 13/13] x86/vsyscall/64: Use X86_PF constants in the simulated #PF error code Andy Lutomirski
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20181127144603.GA3130@linux.intel.com \
--to=sean.j.christopherson@intel.com \
--cc=bp@alien8.de \
--cc=dave.hansen@linux.intel.com \
--cc=linux-kernel@vger.kernel.org \
--cc=luto@kernel.org \
--cc=peterz@infradead.org \
--cc=x86@kernel.org \
--cc=yu-cheng.yu@intel.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).