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x86emul: catch exceptions occurring in stubs

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Before adding more use of stubs cloned from decoded guest insns, guard
ourselves against mistakes there: Should an exception (with the
noteworthy exception of #PF) occur inside the stub, forward it to the
guest.

Since the exception fixup table entry can't encode the address of the
faulting insn itself, attach it to the return address instead. This at
once provides a convenient place to hand the exception information
back: The return address is being overwritten by it before branching to
the recovery code.

Take the opportunity and (finally!) add symbol resolution to the
respective log messages (the new one is intentionally not being coded
that way, as it covers stub addresses only, which don't have symbols
associated).

Also take the opportunity and make search_one_extable() static again.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
This commit is contained in:
Jan Beulich committed 2017-02-16 18:11:42 +01:00
1 parent 8c935f5ff1
commit 79903e50db
4 files changed
+146 -36
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xen/arch/x86/extable.c
+81 -4
View File
@@ -6,6 +6,7 @@
#include <xen/sort.h>
#include <xen/spinlock.h>
#include <asm/uaccess.h>
#include <xen/domain_page.h>
#include <xen/virtual_region.h>
#include <xen/livepatch.h>
@@ -62,7 +63,7 @@ void __init sort_exception_tables(void)
sort_exception_table(__start___pre_ex_table, __stop___pre_ex_table);
}
unsigned long
static unsigned long
search_one_extable(const struct exception_table_entry *first,
const struct exception_table_entry *last,
unsigned long value)
@@ -85,16 +86,92 @@ search_one_extable(const struct exception_table_entry *first,
}
unsigned long
search_exception_table(unsigned long addr)
search_exception_table(const struct cpu_user_regs *regs)
{
const struct virtual_region *region = find_text_region(addr);
const struct virtual_region *region = find_text_region(regs->rip);
unsigned long stub = this_cpu(stubs.addr);
if ( region && region->ex )
return search_one_extable(region->ex, region->ex_end - 1, addr);
return search_one_extable(region->ex, region->ex_end - 1, regs->rip);
if ( regs->rip >= stub + STUB_BUF_SIZE / 2 &&
regs->rip < stub + STUB_BUF_SIZE &&
regs->rsp > (unsigned long)regs &&
regs->rsp < (unsigned long)get_cpu_info() )
{
unsigned long retptr = *(unsigned long *)regs->rsp;
region = find_text_region(retptr);
retptr = region && region->ex
? search_one_extable(region->ex, region->ex_end - 1, retptr)
: 0;
if ( retptr )
{
/*
* Put trap number and error code on the stack (in place of the
* original return address) for recovery code to pick up.
*/
union stub_exception_token token = {
.fields.ec = regs->error_code,
.fields.trapnr = regs->entry_vector,
};
*(unsigned long *)regs->rsp = token.raw;
return retptr;
}
}
return 0;
}
#ifndef NDEBUG
static int __init stub_selftest(void)
{
static const struct {
uint8_t opc[4];
uint64_t rax;
union stub_exception_token res;
} tests[] __initconst = {
{ .opc = { 0x0f, 0xb9, 0xc3, 0xc3 }, /* ud1 */
.res.fields.trapnr = TRAP_invalid_op },
{ .opc = { 0x90, 0x02, 0x00, 0xc3 }, /* nop; add (%rax),%al */
.rax = 0x0123456789abcdef,
.res.fields.trapnr = TRAP_gp_fault },
{ .opc = { 0x02, 0x04, 0x04, 0xc3 }, /* add (%rsp,%rax),%al */
.rax = 0xfedcba9876543210,
.res.fields.trapnr = TRAP_stack_error },
};
unsigned long addr = this_cpu(stubs.addr) + STUB_BUF_SIZE / 2;
unsigned int i;
for ( i = 0; i < ARRAY_SIZE(tests); ++i )
{
uint8_t *ptr = map_domain_page(_mfn(this_cpu(stubs.mfn))) +
(addr & ~PAGE_MASK);
unsigned long res = ~0;
memset(ptr, 0xcc, STUB_BUF_SIZE / 2);
memcpy(ptr, tests[i].opc, ARRAY_SIZE(tests[i].opc));
unmap_domain_page(ptr);
asm volatile ( "call *%[stb]\n"
".Lret%=:\n\t"
".pushsection .fixup,\"ax\"\n"
".Lfix%=:\n\t"
"pop %[exn]\n\t"
"jmp .Lret%=\n\t"
".popsection\n\t"
_ASM_EXTABLE(.Lret%=, .Lfix%=)
: [exn] "+m" (res)
: [stb] "rm" (addr), "a" (tests[i].rax));
ASSERT(res == tests[i].res.raw);
}
return 0;
}
__initcall(stub_selftest);
#endif
unsigned long
search_pre_exception_table(struct cpu_user_regs *regs)
{
xen/arch/x86/traps.c
+15 -15
View File
@@ -798,15 +798,6 @@ void do_trap(struct cpu_user_regs *regs)
return;
}
if ( likely((fixup = search_exception_table(regs->rip)) != 0) )
{
dprintk(XENLOG_ERR, "Trap %d: %p -> %p\n",
trapnr, _p(regs->rip), _p(fixup));
this_cpu(last_extable_addr) = regs->rip;
regs->rip = fixup;
return;
}
if ( ((trapnr == TRAP_copro_error) || (trapnr == TRAP_simd_error)) &&
system_state >= SYS_STATE_active && has_hvm_container_vcpu(curr) &&
curr->arch.hvm_vcpu.fpu_exception_callback )
@@ -816,6 +807,15 @@ void do_trap(struct cpu_user_regs *regs)
return;
}
if ( likely((fixup = search_exception_table(regs)) != 0) )
{
dprintk(XENLOG_ERR, "Trap %u: %p [%ps] -> %p\n",
trapnr, _p(regs->rip), _p(regs->rip), _p(fixup));
this_cpu(last_extable_addr) = regs->rip;
regs->rip = fixup;
return;
}
hardware_trap:
if ( debugger_trap_fatal(trapnr, regs) )
return;
@@ -1185,7 +1185,7 @@ void do_invalid_op(struct cpu_user_regs *regs)
}
die:
if ( (fixup = search_exception_table(regs->rip)) != 0 )
if ( (fixup = search_exception_table(regs)) != 0 )
{
this_cpu(last_extable_addr) = regs->rip;
regs->rip = fixup;
@@ -1515,7 +1515,7 @@ void do_page_fault(struct cpu_user_regs *regs)
if ( pf_type != real_fault )
return;
if ( likely((fixup = search_exception_table(regs->rip)) != 0) )
if ( likely((fixup = search_exception_table(regs)) != 0) )
{
perfc_incr(copy_user_faults);
if ( unlikely(regs->error_code & PFEC_reserved_bit) )
@@ -3462,10 +3462,10 @@ void do_general_protection(struct cpu_user_regs *regs)
gp_in_kernel:
if ( likely((fixup = search_exception_table(regs->rip)) != 0) )
if ( likely((fixup = search_exception_table(regs)) != 0) )
{
dprintk(XENLOG_INFO, "GPF (%04x): %p -> %p\n",
regs->error_code, _p(regs->rip), _p(fixup));
dprintk(XENLOG_INFO, "GPF (%04x): %p [%ps] -> %p\n",
regs->error_code, _p(regs->rip), _p(regs->rip), _p(fixup));
this_cpu(last_extable_addr) = regs->rip;
regs->rip = fixup;
return;
@@ -3741,7 +3741,7 @@ void do_debug(struct cpu_user_regs *regs)
* watchpoint set on it. No need to bump EIP; the only faulting
* trap is an instruction breakpoint, which can't happen to us.
*/
WARN_ON(!search_exception_table(regs->rip));
WARN_ON(!search_exception_table(regs));
}
goto out;
}
xen/arch/x86/x86_emulate/x86_emulate.c
+41 -16
View File
@@ -604,14 +604,43 @@ do{ asm volatile ( \
#define __emulate_1op_8byte(_op, _dst, _eflags)
#endif /* __i386__ */
#ifdef __XEN__
# define invoke_stub(pre, post, constraints...) do { \
union stub_exception_token res_ = { .raw = ~0 }; \
asm volatile ( pre "\n\tcall *%[stub]\n\t" post "\n" \
".Lret%=:\n\t" \
".pushsection .fixup,\"ax\"\n" \
".Lfix%=:\n\t" \
"pop %[exn]\n\t" \
"jmp .Lret%=\n\t" \
".popsection\n\t" \
_ASM_EXTABLE(.Lret%=, .Lfix%=) \
: [exn] "+g" (res_), constraints, \
[stub] "rm" (stub.func) ); \
if ( unlikely(~res_.raw) ) \
{ \
gprintk(XENLOG_WARNING, \
"exception %u (ec=%04x) in emulation stub (line %u)\n", \
res_.fields.trapnr, res_.fields.ec, __LINE__); \
gprintk(XENLOG_INFO, "stub: %"__stringify(MAX_INST_LEN)"ph\n", \
stub.func); \
generate_exception_if(res_.fields.trapnr == EXC_UD, EXC_UD); \
domain_crash(current->domain); \
goto cannot_emulate; \
} \
} while (0)
#else
# define invoke_stub(pre, post, constraints...) \
asm volatile ( pre "\n\tcall *%[stub]\n\t" post \
: constraints, [stub] "rm" (stub.func) )
#endif
#define emulate_stub(dst, src...) do { \
unsigned long tmp; \
asm volatile ( _PRE_EFLAGS("[efl]", "[msk]", "[tmp]") \
"call *%[stub];" \
_POST_EFLAGS("[efl]", "[msk]", "[tmp]") \
: dst, [tmp] "=&r" (tmp), [efl] "+g" (_regs._eflags) \
: [stub] "r" (stub.func), \
[msk] "i" (EFLAGS_MASK), ## src ); \
invoke_stub(_PRE_EFLAGS("[efl]", "[msk]", "[tmp]"), \
_POST_EFLAGS("[efl]", "[msk]", "[tmp]"), \
dst, [tmp] "=&r" (tmp), [efl] "+g" (_regs._eflags) \
: [msk] "i" (EFLAGS_MASK), ## src); \
} while (0)
/* Fetch next part of the instruction being emulated. */
@@ -858,8 +887,7 @@ do { \
unsigned int nr_ = sizeof((uint8_t[]){ bytes }); \
fic.insn_bytes = nr_; \
memcpy(get_stub(stub), ((uint8_t[]){ bytes, 0xc3 }), nr_ + 1); \
asm volatile ( "call *%[stub]" : "+m" (fic) : \
[stub] "rm" (stub.func) ); \
invoke_stub("", "", "=m" (fic) : "m" (fic)); \
put_stub(stub); \
} while (0)
@@ -869,14 +897,11 @@ do { \
unsigned long tmp_; \
fic.insn_bytes = nr_; \
memcpy(get_stub(stub), ((uint8_t[]){ bytes, 0xc3 }), nr_ + 1); \
asm volatile ( _PRE_EFLAGS("[eflags]", "[mask]", "[tmp]") \
"call *%[func];" \
_POST_EFLAGS("[eflags]", "[mask]", "[tmp]") \
: [eflags] "+g" (_regs._eflags), \
[tmp] "=&r" (tmp_), "+m" (fic) \
: [func] "rm" (stub.func), \
[mask] "i" (X86_EFLAGS_ZF|X86_EFLAGS_PF| \
X86_EFLAGS_CF) ); \
invoke_stub(_PRE_EFLAGS("[eflags]", "[mask]", "[tmp]"), \
_POST_EFLAGS("[eflags]", "[mask]", "[tmp]"), \
[eflags] "+g" (_regs._eflags), [tmp] "=&r" (tmp_), \
"+m" (fic) \
: [mask] "i" (X86_EFLAGS_ZF|X86_EFLAGS_PF|X86_EFLAGS_CF)); \
put_stub(stub); \
} while (0)
xen/include/asm-x86/uaccess.h
+9 -1
View File
@@ -274,7 +274,15 @@ extern struct exception_table_entry __stop___ex_table[];
extern struct exception_table_entry __start___pre_ex_table[];
extern struct exception_table_entry __stop___pre_ex_table[];
extern unsigned long search_exception_table(unsigned long);
union stub_exception_token {
struct {
uint16_t ec;
uint8_t trapnr;
} fields;
unsigned long raw;
};
extern unsigned long search_exception_table(const struct cpu_user_regs *regs);
extern void sort_exception_tables(void);
extern void sort_exception_table(struct exception_table_entry *start,
const struct exception_table_entry *stop);
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