File 57d18642-x86-segment-Bounds-check-accesses-to-emulation-ctxt-seg_reg.patch of Package xen.4507
References: bsc#995792
# Commit 4fa0105d95be6e7145a1f6fd1036ccd43976228c
# Date 2016-09-08 16:39:46 +0100
# Author Andrew Cooper <andrew.cooper3@citrix.com>
# Committer Andrew Cooper <andrew.cooper3@citrix.com>
x86/segment: Bounds check accesses to emulation ctxt->seg_reg[]
HVM HAP codepaths have space for all segment registers in the seg_reg[]
cache (with x86_seg_none still risking an array overrun), while the shadow
codepaths only have space for the user segments.
Range check the input segment of *_get_seg_reg() against the size of the array
used to cache the results, to avoid overruns in the case that the callers
don't filter their input suitably.
Subsume the is_x86_user_segment(seg) checks from the shadow code, which were
an incomplete attempt at range checking, and are now superceeded. Make
hvm_get_seg_reg() static, as it is not used outside of shadow/common.c
No functional change, but far easier to reason that no overflow is possible.
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
# Commit 4c47c47938ea24c73d9459f9f0b6923513772b5d
# Date 2016-09-09 15:31:01 +0100
# Author Andrew Cooper <andrew.cooper3@citrix.com>
# Committer Andrew Cooper <andrew.cooper3@citrix.com>
xen/x86: Fix build with clang following c/s 4fa0105
https://travis-ci.org/xen-project/xen/jobs/158494027#L2344
Clang complains:
emulate.c:2016:14: error: comparison of unsigned enum expression < 0
is always false [-Werror,-Wtautological-compare]
if ( seg < 0 || seg >= ARRAY_SIZE(hvmemul_ctxt->seg_reg) )
~~~ ^ ~
Clang is wrong to raise a warning like this. The signed-ness of an enum is
implementation defined in C, and robust code must not assume the choices made
by the compiler.
In this case, dropping the < 0 check creates a latent bug which would result
in an array underflow when compiled with a compiler which chooses a signed
enum.
Work around the bug by explicitly pulling seg into an unsigned integer, and
only perform the upper bounds check.
No functional change.
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -436,6 +436,8 @@ static int hvmemul_virtual_to_linear(
*reps = min_t(unsigned long, *reps, 4096);
reg = hvmemul_get_seg_reg(seg, hvmemul_ctxt);
+ if ( IS_ERR(reg) )
+ return -PTR_ERR(reg);
if ( (hvmemul_ctxt->ctxt.regs->eflags & X86_EFLAGS_DF) && (*reps > 1) )
{
@@ -926,6 +928,10 @@ static int hvmemul_read_segment(
struct hvm_emulate_ctxt *hvmemul_ctxt =
container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
struct segment_register *sreg = hvmemul_get_seg_reg(seg, hvmemul_ctxt);
+
+ if ( IS_ERR(sreg) )
+ return -PTR_ERR(sreg);
+
memcpy(reg, sreg, sizeof(struct segment_register));
return X86EMUL_OKAY;
}
@@ -939,6 +945,9 @@ static int hvmemul_write_segment(
container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
struct segment_register *sreg = hvmemul_get_seg_reg(seg, hvmemul_ctxt);
+ if ( IS_ERR(sreg) )
+ return -PTR_ERR(sreg);
+
memcpy(sreg, reg, sizeof(struct segment_register));
__set_bit(seg, &hvmemul_ctxt->seg_reg_dirty);
@@ -1302,11 +1311,20 @@ void hvm_emulate_writeback(
}
}
+/*
+ * Callers which pass a known in-range x86_segment can rely on the return
+ * pointer being valid. Other callers must explicitly check for errors.
+ */
struct segment_register *hvmemul_get_seg_reg(
enum x86_segment seg,
struct hvm_emulate_ctxt *hvmemul_ctxt)
{
- if ( !__test_and_set_bit(seg, &hvmemul_ctxt->seg_reg_accessed) )
- hvm_get_segment_register(current, seg, &hvmemul_ctxt->seg_reg[seg]);
- return &hvmemul_ctxt->seg_reg[seg];
+ unsigned int idx = seg;
+
+ if ( idx >= ARRAY_SIZE(hvmemul_ctxt->seg_reg) )
+ return ERR_PTR(-X86EMUL_UNHANDLEABLE);
+
+ if ( !__test_and_set_bit(idx, &hvmemul_ctxt->seg_reg_accessed) )
+ hvm_get_segment_register(current, idx, &hvmemul_ctxt->seg_reg[idx]);
+ return &hvmemul_ctxt->seg_reg[idx];
}
--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -120,12 +120,22 @@ __initcall(shadow_audit_key_init);
/* x86 emulator support for the shadow code
*/
+/*
+ * Callers which pass a known in-range x86_segment can rely on the return
+ * pointer being valid. Other callers must explicitly check for errors.
+ */
struct segment_register *hvm_get_seg_reg(
enum x86_segment seg, struct sh_emulate_ctxt *sh_ctxt)
{
- struct segment_register *seg_reg = &sh_ctxt->seg_reg[seg];
- if ( !__test_and_set_bit(seg, &sh_ctxt->valid_seg_regs) )
- hvm_get_segment_register(current, seg, seg_reg);
+ unsigned int idx = seg;
+ struct segment_register *seg_reg;
+
+ if ( idx >= ARRAY_SIZE(sh_ctxt->seg_reg) )
+ return ERR_PTR(-X86EMUL_UNHANDLEABLE);
+
+ seg_reg = &sh_ctxt->seg_reg[idx];
+ if ( !__test_and_set_bit(idx, &sh_ctxt->valid_seg_regs) )
+ hvm_get_segment_register(current, idx, seg_reg);
return seg_reg;
}
@@ -140,14 +150,9 @@ static int hvm_translate_linear_addr(
struct segment_register *reg;
int okay;
- /*
- * Can arrive here with non-user segments. However, no such cirucmstance
- * is part of a legitimate pagetable update, so fail the emulation.
- */
- if ( !is_x86_user_segment(seg) )
- return X86EMUL_UNHANDLEABLE;
-
reg = hvm_get_seg_reg(seg, sh_ctxt);
+ if ( IS_ERR(reg) )
+ return -PTR_ERR(reg);
okay = hvm_virtual_to_linear_addr(
seg, reg, offset, bytes, access_type, sh_ctxt->ctxt.addr_size, paddr);
@@ -249,9 +254,6 @@ hvm_emulate_write(enum x86_segment seg,
unsigned long addr;
int rc;
- if ( !is_x86_user_segment(seg) )
- return X86EMUL_UNHANDLEABLE;
-
/* How many emulations could we save if we unshadowed on stack writes? */
if ( seg == x86_seg_ss )
perfc_incr(shadow_fault_emulate_stack);
@@ -279,9 +281,6 @@ hvm_emulate_cmpxchg(enum x86_segment seg
unsigned long addr, old[2], new[2];
int rc;
- if ( !is_x86_user_segment(seg) )
- return X86EMUL_UNHANDLEABLE;
-
rc = hvm_translate_linear_addr(
seg, offset, bytes, hvm_access_write, sh_ctxt, &addr);
if ( rc )
--- a/xen/include/asm-x86/hvm/emulate.h
+++ b/xen/include/asm-x86/hvm/emulate.h
@@ -13,6 +13,7 @@
#define __ASM_X86_HVM_EMULATE_H__
#include <xen/config.h>
+#include <xen/err.h>
#include <asm/x86_emulate.h>
struct hvm_emulate_ctxt {
