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x86: Make sure we can boot in the case the BDA contains pure garbage

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On non-BIOS platforms it is possible that the BIOS data area contains
garbage instead of being zeroed or something equivalent (firmware
people: we are talking of 1.5K here, so please do the sane thing.)

We need on the order of 20-30K of low memory in order to boot, which
may grow up to < 64K in the future.  We probably want to avoid the
lowest of the low memory.  At the same time, it seems extremely
unlikely that a legitimate EBDA would ever reach down to the 128K
(which would require it to be over half a megabyte in size.)  Thus,
pick 128K as the cutoff for "this is insane, ignore."  We may still
end up reserving a bunch of extra memory on the low megabyte, but that
is not really a major issue these days.  In the worst case we lose
512K of RAM.

This code really should be merged with trim_bios_range() in
arch/x86/kernel/setup.c, but that is a bigger patch for a later merge
window.

Reported-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Matt Fleming <matt.fleming@intel.com>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/n/tip-oebml055yyfm8yxmria09rja@git.kernel.org
This commit is contained in:
H. Peter Anvin 2013-02-27 12:46:40 -08:00
parent 058e7b5814
commit 7c10093692
1 changed files with 34 additions and 19 deletions
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53
arch/x86/kernel/head.c
View file

@ -5,8 +5,6 @@
#include <asm/setup.h>
#include <asm/bios_ebda.h>
#define BIOS_LOWMEM_KILOBYTES 0x413
/*
* The BIOS places the EBDA/XBDA at the top of conventional
* memory, and usually decreases the reported amount of
@ -16,17 +14,30 @@
* chipset: reserve a page before VGA to prevent PCI prefetch
* into it (errata #56). Usually the page is reserved anyways,
* unless you have no PS/2 mouse plugged in.
*
* This functions is deliberately very conservative. Losing
* memory in the bottom megabyte is rarely a problem, as long
* as we have enough memory to install the trampoline. Using
* memory that is in use by the BIOS or by some DMA device
* the BIOS didn't shut down *is* a big problem.
*/
#define BIOS_LOWMEM_KILOBYTES 0x413
#define LOWMEM_CAP 0x9f000U /* Absolute maximum */
#define INSANE_CUTOFF 0x20000U /* Less than this = insane */
void __init reserve_ebda_region(void)
{
unsigned int lowmem, ebda_addr;
/* To determine the position of the EBDA and the */
/* end of conventional memory, we need to look at */
/* the BIOS data area. In a paravirtual environment */
/* that area is absent. We'll just have to assume */
/* that the paravirt case can handle memory setup */
/* correctly, without our help. */
/*
* To determine the position of the EBDA and the
* end of conventional memory, we need to look at
* the BIOS data area. In a paravirtual environment
* that area is absent. We'll just have to assume
* that the paravirt case can handle memory setup
* correctly, without our help.
*/
if (paravirt_enabled())
return;
@ -37,19 +48,23 @@ void __init reserve_ebda_region(void)
/* start of EBDA area */
ebda_addr = get_bios_ebda();
/* Fixup: bios puts an EBDA in the top 64K segment */
/* of conventional memory, but does not adjust lowmem. */
if ((lowmem - ebda_addr) <= 0x10000)
lowmem = ebda_addr;
/*
* Note: some old Dells seem to need 4k EBDA without
* reporting so, so just consider the memory above 0x9f000
* to be off limits (bugzilla 2990).
*/
/* Fixup: bios does not report an EBDA at all. */
/* Some old Dells seem to need 4k anyhow (bugzilla 2990) */
if ((ebda_addr == 0) && (lowmem >= 0x9f000))
lowmem = 0x9f000;
/* If the EBDA address is below 128K, assume it is bogus */
if (ebda_addr < INSANE_CUTOFF)
ebda_addr = LOWMEM_CAP;
/* Paranoia: should never happen, but... */
if ((lowmem == 0) || (lowmem >= 0x100000))
lowmem = 0x9f000;
/* If lowmem is less than 128K, assume it is bogus */
if (lowmem < INSANE_CUTOFF)
lowmem = LOWMEM_CAP;
/* Use the lower of the lowmem and EBDA markers as the cutoff */
lowmem = min(lowmem, ebda_addr);
lowmem = min(lowmem, LOWMEM_CAP); /* Absolute cap */
/* reserve all memory between lowmem and the 1MB mark */
memblock_reserve(lowmem, 0x100000 - lowmem);