Re: [PATCH] x86/microcode/intel: Use correct buffer size for saving microcode data

[Borislav Petkov]

On Thu, Jan 05, 2017 at 11:47:56PM +0000, Junichi Nomura wrote:
> The problem can be reproduced either with the one in RHEL6 (
> microcode-20151106.dat) or with the latest blob from Intel,
> i.e. microcode-20161104.tgz.

Ok, thanks, I was able to reproduce and now I see what's going on. I've
expanded your commit message to explain the situation better, see below.

Thanks again for that good catch!

---
From: Junichi Nomura <j-nomura@xxxxxxxxxxxxx>
Date: Thu, 5 Jan 2017 04:45:18 +0000
Subject: [PATCH] x86/microcode/intel: Use correct buffer size for saving
 microcode data

In generic_load_microcode(), curr_mc_size is the size of the last
allocated buffer and since we have this performance "optimization"
there to vmalloc a new buffer only when the current one is bigger,
curr_mc_size ends up becoming the size of the biggest buffer we've seen
so far.

However, we end up saving the microcode patch which matches our CPU
and its size is not curr_mc_size but the respective mc_size during the
iteration while we're staring at it.

So save that mc_size into a separate variable and use it to store the
previously found microcode buffer.

Without this fix, we could get oops like this:

  BUG: unable to handle kernel paging request at ffffc9000e30f000
  IP: __memcpy+0x12/0x20
  ...
  Call Trace:
  ? kmemdup+0x43/0x60
  __alloc_microcode_buf+0x44/0x70
  save_microcode_patch+0xd4/0x150
  generic_load_microcode+0x1b8/0x260
  request_microcode_user+0x15/0x20
  microcode_write+0x91/0x100
  __vfs_write+0x34/0x120
  vfs_write+0xc1/0x130
  SyS_write+0x56/0xc0
  do_syscall_64+0x6c/0x160
  entry_SYSCALL64_slow_path+0x25/0x25

Signed-off-by: Jun'ichi Nomura <j-nomura@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/4f33cbfd-44f2-9bed-3b66-7446cd14256f@xxxxxxxxxxxxx
Fixes: 06b8534cb728 ("x86/microcode: Rework microcode loading")
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
---
 arch/x86/kernel/cpu/microcode/intel.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
index 943486589757..3f329b74e040 100644
--- a/arch/x86/kernel/cpu/microcode/intel.c
+++ b/arch/x86/kernel/cpu/microcode/intel.c
@@ -823,7 +823,7 @@ static enum ucode_state generic_load_microcode(int cpu, void *data, size_t size,
 	u8 *ucode_ptr = data, *new_mc = NULL, *mc = NULL;
 	int new_rev = uci->cpu_sig.rev;
 	unsigned int leftover = size;
-	unsigned int curr_mc_size = 0;
+	unsigned int curr_mc_size = 0, new_mc_size = 0;
 	unsigned int csig, cpf;
 
 	while (leftover) {
@@ -864,6 +864,7 @@ static enum ucode_state generic_load_microcode(int cpu, void *data, size_t size,
 			vfree(new_mc);
 			new_rev = mc_header.rev;
 			new_mc  = mc;
+			new_mc_size = mc_size;
 			mc = NULL;	/* trigger new vmalloc */
 		}
 
@@ -889,7 +890,7 @@ static enum ucode_state generic_load_microcode(int cpu, void *data, size_t size,
 	 * permanent memory. So it will be loaded early when a CPU is hot added
 	 * or resumes.
 	 */
-	save_mc_for_early(new_mc, curr_mc_size);
+	save_mc_for_early(new_mc, new_mc_size);
 
 	pr_debug("CPU%d found a matching microcode update with version 0x%x (current=0x%x)\n",
 		 cpu, new_rev, uci->cpu_sig.rev);
-- 
2.11.0

-- 
Regards/Gruss,
    Boris.

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