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

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

diff --git a/arch/x86/kernel/head.c b/arch/x86/kernel/head.c
index 48d9d4e..992f442 100644 (file)
--- a/arch/x86/kernel/head.c
+++ b/arch/x86/kernel/head.c
@@ -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).
+        */
+
+       /* If the EBDA address is below 128K, assume it is bogus */
+       if (ebda_addr < INSANE_CUTOFF)
+               ebda_addr = LOWMEM_CAP;
 
-       /* 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 lowmem is less than 128K, assume it is bogus */
+       if (lowmem < INSANE_CUTOFF)
+               lowmem = LOWMEM_CAP;
 
-       /* Paranoia: should never happen, but... */
-       if ((lowmem == 0) || (lowmem >= 0x100000))
-               lowmem = 0x9f000;
+       /* 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);