[librm] Do not unconditionally preserve flags across virt_call()

Commit 196f0f2 ("[librm] Convert prot_call() to a real-mode near
call") introduced a regression in which any deliberate modification to
the low 16 bits of the CPU flags (in struct i386_all_regs) would be
overwritten with the original flags value at the time of entry to
prot_call().

The regression arose because the alignment requirements of the
protected-mode stack necessitated the insertion of two bytes of
padding immediately below the prot_call() return address.  The
solution chosen was to extend the existing "pushfl / popfl" pair to
"pushfw;pushfl / popfl;popfw".  The extra "pushfw / popfw" appears at
first glance to be a no-op, but fails to take into account the fact
that the flags restored by popfl may have been deliberately modified
by the protected-mode function.

Fix by replacing "pushfw / popfw" with "pushw %ss / popw %ss".  While
%ss does appear within struct i386_all_regs, any modification to the
stored value has always been ignored by prot_call() anyway.

The most visible symptom of this regression was that SAN booting would
fail since every INT 13 call would be chained to the original INT 13
vector.

Reported-by: Vishvananda Ishaya <vishvananda@gmail.com>
Reported-by: Jamie Thompson <forum.ipxe@jamie-thompson.co.uk>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

diff --git a/src/arch/x86/transitions/librm.S b/src/arch/x86/transitions/librm.S
index bb04ad67df399f8e0e5f082b800c0a790a5a98c4..3a585a9211aaf134c89c7a43968b325b2b6250a0 100644 (file)
--- a/src/arch/x86/transitions/librm.S
+++ b/src/arch/x86/transitions/librm.S
@@ -940,7 +940,7 @@ VC_OFFSET_END:
        .globl virt_call
 virt_call:
        /* Preserve registers, flags and GDT on external RM stack */
-       pushfw  /* padding */
+       pushw   %ss /* padding */
        pushfl
        pushal
        pushw   %gs
@@ -1030,7 +1030,7 @@ vc_rmode:
         */
        addr32 movl -20(%esp), %esp
        popfl
-       popfw   /* padding */
+       popw    %ss /* padding */
 
        /* Return and discard function parameters */
        ret     $( VC_OFFSET_END - VC_OFFSET_PARAMS )