Re: [PATCH] x86/efi-bgrt: Fix kernel panic when mapping BGRT data

[Josh Triplett]

On Thu, Dec 10, 2015 at 10:27:01AM -0800, Sai Praneeth Prakhya wrote:
> From: Sai Praneeth <sai.praneeth.prakhya@xxxxxxxxx>
> 
> Starting with this commit 35eb8b81edd4 ("x86/efi: Build our own page
> table structures") efi regions have a separate page directory called
> "efi_pgd". In order to access any efi region we have to first shift %cr3
> to this page table. In the bgrt code we are trying to copy bgrt_header
> and image, but these regions fall under "EFI_BOOT_SERVICES_DATA"
> and to access these regions we have to shift %cr3 to efi_pgd and not
> doing so will cause page fault as shown below.
> 
> [    0.251599] Last level dTLB entries: 4KB 64, 2MB 0, 4MB 0, 1GB 4
> [    0.259126] Freeing SMP alternatives memory: 32K (ffffffff8230e000 - ffffffff82316000)
> [    0.271803] BUG: unable to handle kernel paging request at fffffffefce35002
> [    0.279740] IP: [<ffffffff821bca49>] efi_bgrt_init+0x144/0x1fd
> [    0.286383] PGD 300f067 PUD 0
> [    0.289879] Oops: 0000 [#1] SMP
> [    0.293566] Modules linked in:
> [    0.297039] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.4.0-rc1-eywa-eywa-built-in-47041+ #2
> [    0.306619] Hardware name: Intel Corporation Skylake Client platform/Skylake Y LPDDR3 RVP3, BIOS SKLSE2R1.R00.B104.B01.1511110114 11/11/2015
> [    0.320925] task: ffffffff820134c0 ti: ffffffff82000000 task.ti: ffffffff82000000
> [    0.329420] RIP: 0010:[<ffffffff821bca49>]  [<ffffffff821bca49>] efi_bgrt_init+0x144/0x1fd
> [    0.338821] RSP: 0000:ffffffff82003f18  EFLAGS: 00010246
> [    0.344852] RAX: fffffffefce35000 RBX: fffffffefce35000 RCX: fffffffefce2b000
> [    0.352952] RDX: 000000008a82b000 RSI: ffffffff8235bb80 RDI: 000000008a835000
> [    0.361050] RBP: ffffffff82003f30 R08: 000000008a865000 R09: ffffffffff202850
> [    0.369149] R10: ffffffff811ad62f R11: 0000000000000000 R12: 0000000000000000
> [    0.377248] R13: ffff88016dbaea40 R14: ffffffff822622c0 R15: ffffffff82003fb0
> [    0.385348] FS:  0000000000000000(0000) GS:ffff88016d800000(0000) knlGS:0000000000000000
> [    0.394533] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [    0.401054] CR2: fffffffefce35002 CR3: 000000000300c000 CR4: 00000000003406f0
> [    0.409153] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [    0.417252] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [    0.425350] Stack:
> [    0.427638]  ffffffffffffffff ffffffff82256900 ffff88016dbaea40 ffffffff82003f40
> [    0.436086]  ffffffff821bbce0 ffffffff82003f88 ffffffff8219c0c2 0000000000000000
> [    0.444533]  ffffffff8219ba4a ffffffff822622c0 0000000000083000 00000000ffffffff
> [    0.452978] Call Trace:
> [    0.455763]  [<ffffffff821bbce0>] efi_late_init+0x9/0xb
> [    0.461697]  [<ffffffff8219c0c2>] start_kernel+0x463/0x47f
> [    0.467928]  [<ffffffff8219ba4a>] ? set_init_arg+0x55/0x55
> [    0.474159]  [<ffffffff8219b120>] ? early_idt_handler_array+0x120/0x120
> [    0.481669]  [<ffffffff8219b5ee>] x86_64_start_reservations+0x2a/0x2c
> [    0.488982]  [<ffffffff8219b72d>] x86_64_start_kernel+0x13d/0x14c
> [    0.495897] Code: 00 41 b4 01 48 8b 78 28 e8 09 36 01 00 48 85 c0 48 89 c3 75 13 48 c7 c7 f8 ac d3 81 31 c0 e8 d7 3b fb fe e9 b5 00 00 00 45 84 e4 <44> 8b 6b 02 74 0d be 06 00 00 00 48 89 df e8 ae 34 0$
> [    0.518151] RIP  [<ffffffff821bca49>] efi_bgrt_init+0x144/0x1fd
> [    0.524888]  RSP <ffffffff82003f18>
> [    0.528851] CR2: fffffffefce35002
> [    0.532615] ---[ end trace 7b06521e6ebf2aea ]---
> [    0.537852] Kernel panic - not syncing: Attempted to kill the idle task!
> 
> As said above one way to fix this bug is to shift %cr3 to efi_pgd but we
> are not doing that way because it leaks inner details of how we switch
> to EFI page tables into a new call site and it also adds duplicate code.
> Instead, we remove the call to efi_lookup_mapped_addr() and always
> perform early_mem*() instead of early_io*() because we want to remap RAM
> regions and not I/O regions.
> 
> We also delete efi_lookup_mapped_addr() because it is impossible to
> use it without also doing the page table switch to efi_pgd.

The original motivation for efi_lookup_mapped_addr came from
early_ioremap printing a warning if used on an address range already
mapped as RAM.  Does early_mem* handle that case correctly without a
warning?  Because not all firmware places the BGRT image in boot
services memory; some firmware places the BGRT image variously in BIOS
reserved memory, ACPI reclaim space, or other strange places.

- Josh Triplett
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