141 lines
5.4 KiB
Diff
141 lines
5.4 KiB
Diff
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Laszlo Ersek <lersek@redhat.com>
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Date: Fri, 7 Apr 2023 16:21:54 +0200
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Subject: [PATCH] grub_dl_set_mem_attrs(): add self-check for the tramp/GOT
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sizes
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On aarch64 UEFI, we currently have a crasher:
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grub_dl_load_core()
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grub_dl_load_core_noinit()
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/* independent allocation: must remain writable */
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mod = grub_zalloc();
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/* allocates module image with incorrect tail alignment */
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grub_dl_load_segments()
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/* write-protecting the module image makes "mod" read-only! */
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grub_dl_set_mem_attrs()
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grub_update_mem_attrs()
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grub_dl_init()
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/* page fault, crash */
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mod->next = ...;
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- Commit 887f1d8fa976 ("modules: load module sections at page-aligned
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addresses", 2023-02-08) forgot to page-align the allocation of the
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trampolines and GOT areas of grub2 modules, in grub_dl_load_segments().
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- Commit ad1b904d325b ("nx: set page permissions for loaded modules.",
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2023-02-08) calculated a common bounding box for the trampolines and GOT
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areas in grub_dl_set_mem_attrs(), rounded the box size up to a whole
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multiple of EFI page size ("arch_addralign"), and write-protected the
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resultant page range.
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Consequently, grub_dl_load_segments() places the module image in memory
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such that its tail -- the end of the trampolines and GOT areas -- lands at
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the head of a page whose tail in turn contains independent memory
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allocations, such as "mod". grub_dl_set_mem_attrs() will then unwittingly
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write-protect these other allocations too.
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But "mod" must remain writable: we assign "mod->next" in grub_dl_init()
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subsequently. Currently we crash there with a page fault / permission
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fault.
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(The crash is not trivial to hit: the tramp/GOT areas are irrelevant on
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x86_64, plus the page protection depends on the UEFI platform firmware
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providing EFI_MEMORY_ATTRIBUTE_PROTOCOL. In practice, the crash is
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restricted to aarch64 edk2 (ArmVirtQemu) builds containing commit
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1c4dfadb4611, "ArmPkg/CpuDxe: Implement EFI memory attributes protocol",
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2023-03-16.)
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Example log before the patch:
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> kern/dl.c:736: updating attributes for GOT and trampolines ("video_fb")
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> kern/efi/mm.c:927: set +rx -w on 0x13b88b000-0x13b88bfff before:rwx after:r-x
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> kern/dl.c:744: done updating module memory attributes for "video_fb"
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> kern/dl.c:639: flushing 0xe4f0 bytes at 0x13b87d000
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> kern/arm64/cache.c:42: D$ line size: 64
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> kern/arm64/cache.c:43: I$ line size: 64
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> kern/dl.c:839: module name: video_fb
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> kern/dl.c:840: init function: 0x0
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> kern/dl.c:865: Initing module video_fb
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>
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> Synchronous Exception at 0x000000013B8A76EC
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> PC 0x00013B8A76EC
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>
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> X0 0x000000013B88B960 X1 0x0000000000000000 X2 0x000000013F93587C X3 0x0000000000000075
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>
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> SP 0x00000000470745C0 ELR 0x000000013B8A76EC SPSR 0x60000205 FPSR 0x00000000
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> ESR 0x9600004F FAR 0x000000013B88B9D0
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>
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> ESR : EC 0x25 IL 0x1 ISS 0x0000004F
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>
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> Data abort: Permission fault, third level
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Note the following:
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- The whole 4K page at 0x1_3B88_B000 is write-protected.
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- The "video_fb" module actually lives at [0x1_3B87_D000, 0x1_3B88_B4F0)
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-- left-inclusive, right-exclusive --; that is, in the last page (at
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0x1_3B88_B000), it only occupies the first 0x4F0 bytes.
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- The instruction at 0x1_3B8A_76EC faults. Not shown here, but it is a
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store instruction, which writes to the field at offset 0x70 of the
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structure pointed-to by the X0 register. This is the "mod->next"
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assignment from grub_dl_init().
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- The faulting address is therefore (X0 + 0x70), i.e., 0x1_3B88_B9D0. This
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is indeed the value held in the FAR register.
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- The faulting address 0x1_3B88_B9D0 falls in the above-noted page (at
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0x1_3B88_B000), namely at offset 0x9D0. This is *beyond* the first 0x4F0
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bytes that the very tail of the "video_fb" module occupies at the front
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of that page.
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For now, add a self-check that reports this bug (and prevents the crash by
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skipping the write protection).
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Example log after the patch:
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> kern/dl.c:742:BUG: trying to protect pages outside of module allocation
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> ("video_fb"): module base 0x13b87d000, size 0xe4f0; tramp/GOT base
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> 0x13b88b000, size 0x1000
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Signed-off-by: Laszlo Ersek <lersek@redhat.com>
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---
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grub-core/kern/dl.c | 11 ++++++++++-
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1 file changed, 10 insertions(+), 1 deletion(-)
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diff --git a/grub-core/kern/dl.c b/grub-core/kern/dl.c
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index a97f4a8b13..3b66fa410e 100644
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--- a/grub-core/kern/dl.c
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+++ b/grub-core/kern/dl.c
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@@ -682,7 +682,7 @@ grub_dl_set_mem_attrs (grub_dl_t mod, void *ehdr)
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#if !defined (__i386__) && !defined (__x86_64__) && !defined(__riscv)
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grub_size_t arch_addralign = grub_arch_dl_min_alignment ();
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grub_addr_t tgaddr;
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- grub_uint64_t tgsz;
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+ grub_size_t tgsz;
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#endif
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grub_dprintf ("modules", "updating memory attributes for \"%s\"\n",
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@@ -736,6 +736,15 @@ grub_dl_set_mem_attrs (grub_dl_t mod, void *ehdr)
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grub_dprintf ("modules",
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"updating attributes for GOT and trampolines (\"%s\")\n",
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mod->name);
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+ if (tgaddr < (grub_addr_t)mod->base ||
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+ tgsz > (grub_addr_t)-1 - tgaddr ||
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+ tgaddr + tgsz > (grub_addr_t)mod->base + mod->sz)
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+ return grub_error (GRUB_ERR_BUG,
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+ "BUG: trying to protect pages outside of module "
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+ "allocation (\"%s\"): module base %p, size 0x%"
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+ PRIxGRUB_SIZE "; tramp/GOT base 0x%" PRIxGRUB_ADDR
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+ ", size 0x%" PRIxGRUB_SIZE,
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+ mod->name, mod->base, mod->sz, tgaddr, tgsz);
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grub_update_mem_attrs (tgaddr, tgsz, GRUB_MEM_ATTR_R|GRUB_MEM_ATTR_X,
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GRUB_MEM_ATTR_W);
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}
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