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 <l...@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
>
