[romprefix] Further sanity checks for the PCI 3 runtime segment address

This extends the sanity checks on the runtime segment address provided
in %bx, first implemented in commit 5600955.

We now allow the ROM to be placed anywhere above a000:0000 (rather
than c000:0000, as before), since this is the region allowed by the
PCI 3 spec.  If the BIOS asks us to place the runtime image such that
it would overlap with the init-time image (which is explicitly
prohibited by the PCI 3 spec), then we assume that the BIOS is faulty
and ignore the provided runtime segment address.

Testing on a SuperMicro BIOS providing overlapping segment addresses
shows that ignoring the provided runtime segment address is safe to do
in these circumstances.

diff --git a/src/arch/i386/prefix/romprefix.S b/src/arch/i386/prefix/romprefix.S
index 3351494..9407e64 100644 (file)
--- a/src/arch/i386/prefix/romprefix.S
+++ b/src/arch/i386/prefix/romprefix.S
@@ -190,11 +190,11 @@ init:
        stc
        movw    $0xb101, %ax
        int     $0x1a
-       jc      1f
+       jc      no_pci3
        cmpl    $PCI_SIGNATURE, %edx
-       jne     1f
+       jne     no_pci3
        testb   %ah, %ah
-       jnz     1f
+       jnz     no_pci3
        movw    $init_message_pci, %si
        xorw    %di, %di
        call    print_message
@@ -205,20 +205,33 @@ init:
        movb    %bl, %al
        call    print_hex_byte
        cmpb    $3, %bh
-       jb      1f
+       jb      no_pci3
        /* PCI >=3.0: leave %gs as-is if sane */
        movw    %gs, %ax
-       cmpw    $0xc000, %ax
-       jae     2f
-       /* PCI 3.0 with insane %gs value: print error and ignore %gs */
+       cmpw    $0xa000, %ax    /* Insane if %gs < 0xa000 */
+       jb      pci3_insane
+       movw    %cs, %bx        /* Sane if %cs == %gs */
+       cmpw    %bx, %ax
+       je      1f
+       movzbw  romheader_size, %cx /* Sane if %cs+len <= %gs */
+       shlw    $5, %cx
+       addw    %cx, %bx
+       cmpw    %bx, %ax
+       jae     1f
+       movw    %cs, %bx        /* Sane if %gs+len <= %cs */
+       addw    %cx, %ax
+       cmpw    %bx, %ax
+       jbe     1f
+pci3_insane: /* PCI 3.0 with insane %gs value: print error and ignore %gs */
        movb    $'!', %al
        call    print_character
        movw    %gs, %ax
        call    print_hex_word
-1:     /* PCI <3.0: set %gs (runtime segment) = %cs (init-time segment) */
+no_pci3:
+       /* PCI <3.0: set %gs (runtime segment) = %cs (init-time segment) */
        pushw   %cs
        popw    %gs
-2:     popl    %edi
+1:     popl    %edi
        popl    %edx
        popl    %ebx