[PATCH v2] x86/fault: Fix wrong signal when vsyscall fails with pkey

From: Jiashuo Liang
Date: Thu Jul 29 2021 - 23:03:10 EST


The function __bad_area_nosemaphore() calls kernelmode_fixup_or_oops() with
parameter "signal" being "pkey", which will send a signal numbered "pkey".

This bug appears when the kernel fail to access user-given memory pages
that are protected by pkey, so it can go through the do_user_addr_fault()
path and pass the !user_mode() check in __bad_area_nosemaphore(). Most
cases will simply run the kernel fixup code to make an -EFAULT. But when
another condition current->thread.sig_on_uaccess_err is met, which is
only used to emulate vsyscall, we will generate the wrong signal.

A new parameter "pkey" is added to kernelmode_fixup_or_oops() to fix it.

Fixes: 5042d40a264c ("x86/fault: Bypass no_context() for implicit kernel faults from usermode")
Signed-off-by: Jiashuo Liang <liangjs@xxxxxxxxxx>
---

Changes in v2:
- Use ARCH_DEFAULT_PKEY instead of 0 for unused pkey parameter.

arch/x86/mm/fault.c | 26 ++++++++++++++++++--------
1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index b2eefdefc108..84a2c8c4af73 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -710,7 +710,8 @@ page_fault_oops(struct pt_regs *regs, unsigned long error_code,

static noinline void
kernelmode_fixup_or_oops(struct pt_regs *regs, unsigned long error_code,
- unsigned long address, int signal, int si_code)
+ unsigned long address, int signal, int si_code,
+ u32 pkey)
{
WARN_ON_ONCE(user_mode(regs));

@@ -735,8 +736,12 @@ kernelmode_fixup_or_oops(struct pt_regs *regs, unsigned long error_code,

set_signal_archinfo(address, error_code);

- /* XXX: hwpoison faults will set the wrong code. */
- force_sig_fault(signal, si_code, (void __user *)address);
+ if (si_code == SEGV_PKUERR) {
+ force_sig_pkuerr((void __user *)address, pkey);
+ } else {
+ /* XXX: hwpoison faults will set the wrong code. */
+ force_sig_fault(signal, si_code, (void __user *)address);
+ }
}

/*
@@ -798,7 +803,8 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
struct task_struct *tsk = current;

if (!user_mode(regs)) {
- kernelmode_fixup_or_oops(regs, error_code, address, pkey, si_code);
+ kernelmode_fixup_or_oops(regs, error_code, address,
+ SIGSEGV, si_code, pkey);
return;
}

@@ -930,7 +936,8 @@ do_sigbus(struct pt_regs *regs, unsigned long error_code, unsigned long address,
{
/* Kernel mode? Handle exceptions or die: */
if (!user_mode(regs)) {
- kernelmode_fixup_or_oops(regs, error_code, address, SIGBUS, BUS_ADRERR);
+ kernelmode_fixup_or_oops(regs, error_code, address,
+ SIGBUS, BUS_ADRERR, ARCH_DEFAULT_PKEY);
return;
}

@@ -1396,7 +1403,8 @@ void do_user_addr_fault(struct pt_regs *regs,
*/
if (!user_mode(regs))
kernelmode_fixup_or_oops(regs, error_code, address,
- SIGBUS, BUS_ADRERR);
+ SIGBUS, BUS_ADRERR,
+ ARCH_DEFAULT_PKEY);
return;
}

@@ -1416,7 +1424,8 @@ void do_user_addr_fault(struct pt_regs *regs,
return;

if (fatal_signal_pending(current) && !user_mode(regs)) {
- kernelmode_fixup_or_oops(regs, error_code, address, 0, 0);
+ kernelmode_fixup_or_oops(regs, error_code, address,
+ 0, 0, ARCH_DEFAULT_PKEY);
return;
}

@@ -1424,7 +1433,8 @@ void do_user_addr_fault(struct pt_regs *regs,
/* Kernel mode? Handle exceptions or die: */
if (!user_mode(regs)) {
kernelmode_fixup_or_oops(regs, error_code, address,
- SIGSEGV, SEGV_MAPERR);
+ SIGSEGV, SEGV_MAPERR,
+ ARCH_DEFAULT_PKEY);
return;
}

--
2.32.0