Re: [patch] x86/crash: fix crash_setup_memmap_entries() out-of-bounds access

[Borislav Petkov <bp@alien8.de>]

Here's an attempt to explain what this fixes:

---
From: Mike Galbraith <efault@gmx.de>
Date: Fri, 16 Apr 2021 14:02:07 +0200
Subject: [PATCH] x86/crash: Fix crash_setup_memmap_entries() out-of-bounds
 access

Commit in Fixes: added support for kexec-ing a kernel on panic using a
new system call. As part of it, it does prepare a memory map for the new
kernel.

However, while doing so, it wrongly accesses memory it has not
allocated: it accesses the first element of the cmem->ranges[] array in
memmap_exclude_ranges() but it has not allocated the memory for it in
crash_setup_memmap_entries(). As KASAN reports:

  BUG: KASAN: vmalloc-out-of-bounds in crash_setup_memmap_entries+0x17e/0x3a0
  Write of size 8 at addr ffffc90000426008 by task kexec/1187

  (gdb) list *crash_setup_memmap_entries+0x17e
  0xffffffff8107cafe is in crash_setup_memmap_entries (arch/x86/kernel/crash.c:322).
  317                                      unsigned long long mend)
  318     {
  319             unsigned long start, end;
  320
  321             cmem->ranges[0].start = mstart;
  322             cmem->ranges[0].end = mend;
  323             cmem->nr_ranges = 1;
  324
  325             /* Exclude elf header region */
  326             start = image->arch.elf_load_addr;
  (gdb)

Make sure the ranges array becomes a single element allocated.

 [ bp: Write a proper commit message. ]

Fixes: dd5f726076cc ("kexec: support for kexec on panic using new system call")
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/725fa3dc1da2737f0f6188a1a9701bead257ea9d.camel@gmx.de
---
 arch/x86/kernel/crash.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
index a8f3af257e26..b1deacbeb266 100644
--- a/arch/x86/kernel/crash.c
+++ b/arch/x86/kernel/crash.c
@@ -337,7 +337,7 @@ int crash_setup_memmap_entries(struct kimage *image, struct boot_params *params)
 	struct crash_memmap_data cmd;
 	struct crash_mem *cmem;
 
-	cmem = vzalloc(sizeof(struct crash_mem));
+	cmem = vzalloc(struct_size(cmem, ranges, 1));
 	if (!cmem)
 		return -ENOMEM;
 
-- 
2.29.2

-- 
Regards/Gruss,
    Boris.

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