File 611a7e38-x86-CET-shstk-WARN-manipulation.patch of Package xen.21116
# Commit 35727551c0703493a2240e967cffc3063b13d49c
# Date 2021-08-16 16:03:20 +0100
# Author Andrew Cooper <andrew.cooper3@citrix.com>
# Committer Andrew Cooper <andrew.cooper3@citrix.com>
x86/cet: Fix shskt manipulation error with BUGFRAME_{warn,run_fn}
This was a clear oversight in the original CET work. The BUGFRAME_run_fn and
BUGFRAME_warn paths update regs->rip without an equivalent adjustment to the
shadow stack, causing IRET to suffer #CP because of the mismatch.
One subtle, and therefore fragile, aspect of extable_shstk_fixup() was that it
required regs->rip to have its old value as a cross-check that the right word
in the shadow stack was being edited.
Rework extable_shstk_fixup() into fixup_exception_return() which takes
ownership of the update to both the regular and shadow stacks, ensuring that
the regs->rip update is ordered correctly.
Use the new fixup_exception_return() for BUGFRAME_run_fn and BUGFRAME_warn to
ensure that the shadow stack is updated too.
Fixes: 209fb9919b50 ("x86/extable: Adjust extable handling to be shadow stack compatible")
Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
# Commit 54c9736382e0d558a6acd820e44185e020131c48
# Date 2021-08-17 12:55:48 +0100
# Author Andrew Cooper <andrew.cooper3@citrix.com>
# Committer Andrew Cooper <andrew.cooper3@citrix.com>
x86/cet: Fix build on newer versions of GCC
Some versions of GCC complain with:
traps.c:405:22: error: 'get_shstk_bottom' defined but not used [-Werror=unused-function]
static unsigned long get_shstk_bottom(unsigned long sp)
^~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
Change #ifdef to if ( IS_ENABLED(...) ) to make the sole user of
get_shstk_bottom() visible to the compiler.
Fixes: 35727551c070 ("x86/cet: Fix shskt manipulation error with BUGFRAME_{warn,run_fn}")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Compile-tested-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -775,53 +775,62 @@ static void do_reserved_trap(struct cpu_
trapnr, vec_name(trapnr), regs->error_code);
}
-static void extable_shstk_fixup(struct cpu_user_regs *regs, unsigned long fixup)
+static void fixup_exception_return(struct cpu_user_regs *regs,
+ unsigned long fixup)
{
- unsigned long ssp, *ptr, *base;
+ if ( IS_ENABLED(CONFIG_XEN_SHSTK) )
+ {
+ unsigned long ssp, *ptr, *base;
- asm ( "rdsspq %0" : "=r" (ssp) : "0" (1) );
- if ( ssp == 1 )
- return;
+ asm ( "rdsspq %0" : "=r" (ssp) : "0" (1) );
+ if ( ssp == 1 )
+ goto shstk_done;
- ptr = _p(ssp);
- base = _p(get_shstk_bottom(ssp));
+ ptr = _p(ssp);
+ base = _p(get_shstk_bottom(ssp));
- for ( ; ptr < base; ++ptr )
- {
- /*
- * Search for %rip. The shstk currently looks like this:
- *
- * ... [Likely pointed to by SSP]
- * %cs [== regs->cs]
- * %rip [== regs->rip]
- * SSP [Likely points to 3 slots higher, above %cs]
- * ... [call tree to this function, likely 2/3 slots]
- *
- * and we want to overwrite %rip with fixup. There are two
- * complications:
- * 1) We cant depend on SSP values, because they won't differ by 3
- * slots if the exception is taken on an IST stack.
- * 2) There are synthetic (unrealistic but not impossible) scenarios
- * where %rip can end up in the call tree to this function, so we
- * can't check against regs->rip alone.
- *
- * Check for both regs->rip and regs->cs matching.
- */
- if ( ptr[0] == regs->rip && ptr[1] == regs->cs )
+ for ( ; ptr < base; ++ptr )
{
- asm ( "wrssq %[fix], %[stk]"
- : [stk] "=m" (ptr[0])
- : [fix] "r" (fixup) );
- return;
+ /*
+ * Search for %rip. The shstk currently looks like this:
+ *
+ * ... [Likely pointed to by SSP]
+ * %cs [== regs->cs]
+ * %rip [== regs->rip]
+ * SSP [Likely points to 3 slots higher, above %cs]
+ * ... [call tree to this function, likely 2/3 slots]
+ *
+ * and we want to overwrite %rip with fixup. There are two
+ * complications:
+ * 1) We cant depend on SSP values, because they won't differ by
+ * 3 slots if the exception is taken on an IST stack.
+ * 2) There are synthetic (unrealistic but not impossible)
+ * scenarios where %rip can end up in the call tree to this
+ * function, so we can't check against regs->rip alone.
+ *
+ * Check for both regs->rip and regs->cs matching.
+ */
+ if ( ptr[0] == regs->rip && ptr[1] == regs->cs )
+ {
+ asm ( "wrssq %[fix], %[stk]"
+ : [stk] "=m" (ptr[0])
+ : [fix] "r" (fixup) );
+ goto shstk_done;
+ }
}
+
+ /*
+ * We failed to locate and fix up the shadow IRET frame. This could
+ * be due to shadow stack corruption, or bad logic above. We cannot
+ * continue executing the interrupted context.
+ */
+ BUG();
+
}
+ shstk_done:
- /*
- * We failed to locate and fix up the shadow IRET frame. This could be
- * due to shadow stack corruption, or bad logic above. We cannot continue
- * executing the interrupted context.
- */
- BUG();
+ /* Fixup the regular stack. */
+ regs->rip = fixup;
}
static bool extable_fixup(struct cpu_user_regs *regs, bool print)
@@ -840,10 +849,7 @@ static bool extable_fixup(struct cpu_use
vec_name(regs->entry_vector), regs->error_code,
_p(regs->rip), _p(regs->rip), _p(fixup));
- if ( IS_ENABLED(CONFIG_XEN_SHSTK) )
- extable_shstk_fixup(regs, fixup);
-
- regs->rip = fixup;
+ fixup_exception_return(regs, fixup);
this_cpu(last_extable_addr) = regs->rip;
return true;
@@ -1127,7 +1133,7 @@ void do_invalid_op(struct cpu_user_regs
void (*fn)(struct cpu_user_regs *) = bug_ptr(bug);
fn(regs);
- regs->rip = (unsigned long)eip;
+ fixup_exception_return(regs, (unsigned long)eip);
return;
}
@@ -1148,7 +1154,7 @@ void do_invalid_op(struct cpu_user_regs
case BUGFRAME_warn:
printk("Xen WARN at %s%s:%d\n", prefix, filename, lineno);
show_execution_state(regs);
- regs->rip = (unsigned long)eip;
+ fixup_exception_return(regs, (unsigned long)eip);
return;
case BUGFRAME_bug:
