From a1a371c468f7238b7826fde55786b02377faf8e2 Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Wed, 21 Nov 2018 15:11:25 -0800 Subject: [PATCH] x86/fault: Decode page fault OOPSes better 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 Cc: Borislav Petkov Cc: Dave Hansen Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Rik van Riel Cc: Thomas Gleixner Cc: Yu-cheng Yu Link: http://lkml.kernel.org/r/11212acb25980cd1b3030875cd9502414fbb214d.1542841400.git.luto@kernel.org Signed-off-by: Ingo Molnar --- 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 ca38bd0..f5efbdb 100644 --- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c @@ -27,6 +27,7 @@ #include /* struct vm86 */ #include /* vma_pkey() */ #include /* efi_recover_from_page_fault()*/ +#include /* store_idt(), ... */ #define CREATE_TRACE_POINTS #include @@ -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"); + 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); } -- 2.7.4