[RFC PATCH 1/5] x86/cet/shstk: Modify ARCH_X86_CET_ALLOC_SHSTK for 32-bit address range

Yu-cheng Yu
2020-05-21T21:17:41Z

Sometimes a 64-bit task might need to have a shadow stack allocated from
within 32-bit address range. One example is selftests/x86/sigreturn.

Currently arch_prctl(ARCH_X86_CET_ALLOC_SHSTK) takes a input parameter for
the desired shadow stack size. Modify it and use bit[0] of the parameter
to indicate the desire to allocate from 32-bit address range.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
---
arch/x86/include/asm/cet.h | 2 +-
arch/x86/include/uapi/asm/prctl.h | 2 ++
arch/x86/kernel/cet.c | 19 ++++++++++++-------
arch/x86/kernel/cet_prctl.c | 6 +++++-
4 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
index f163c805a559..534b02785a39 100644
--- a/arch/x86/include/asm/cet.h
+++ b/arch/x86/include/asm/cet.h
@@ -22,7 +22,7 @@ struct cet_status {
int prctl_cet(int option, u64 arg2);
int cet_setup_shstk(void);
int cet_setup_thread_shstk(struct task_struct *p);
-int cet_alloc_shstk(unsigned long *arg);
+int cet_alloc_shstk(unsigned long *arg, int map_32bit);
void cet_disable_free_shstk(struct task_struct *p);
int cet_verify_rstor_token(bool ia32, unsigned long ssp, unsigned long *new_ssp);
void cet_restore_signal(struct sc_ext *sc);
diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index d962f0ec9ccf..e254c6a21475 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -19,4 +19,6 @@
#define ARCH_X86_CET_LOCK 0x3003
#define ARCH_X86_CET_ALLOC_SHSTK 0x3004

+#define ARCH_X86_CET_ALLOC_SHSTK_32BIT 0x1UL
+
#endif /* _ASM_X86_PRCTL_H */
diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
index 92b8730c0b08..d6f93e1864b2 100644
--- a/arch/x86/kernel/cet.c
+++ b/arch/x86/kernel/cet.c
@@ -57,14 +57,19 @@ static unsigned long cet_get_shstk_addr(void)
return ssp;
}

-static unsigned long alloc_shstk(unsigned long size)
+static unsigned long alloc_shstk(unsigned long size, int map_32bit)
{
struct mm_struct *mm = current->mm;
unsigned long addr, populate;
+ unsigned long map_flags;
+
+ map_flags = MAP_ANONYMOUS | MAP_PRIVATE;
+ if (map_32bit)
+ map_flags |= MAP_32BIT;

down_write(&mm->mmap_sem);
- addr = do_mmap(NULL, 0, size, PROT_READ, MAP_ANONYMOUS | MAP_PRIVATE,
- VM_SHSTK, 0, &populate, NULL);
+ addr = do_mmap(NULL, 0, size, PROT_READ, map_flags, VM_SHSTK, 0,
+ &populate, NULL);
up_write(&mm->mmap_sem);

if (populate)
@@ -147,14 +152,14 @@ static int create_rstor_token(bool ia32, unsigned long ssp,
return 0;
}

-int cet_alloc_shstk(unsigned long *arg)
+int cet_alloc_shstk(unsigned long *arg, int map_32bit)
{
unsigned long len = *arg;
unsigned long addr;
unsigned long token;
unsigned long ssp;

- addr = alloc_shstk(round_up(len, PAGE_SIZE));
+ addr = alloc_shstk(round_up(len, PAGE_SIZE), map_32bit);

if (IS_ERR((void *)addr))
return PTR_ERR((void *)addr);
@@ -185,7 +190,7 @@ int cet_setup_shstk(void)
return -EOPNOTSUPP;

size = round_up(min(rlimit(RLIMIT_STACK), 1UL << 32), PAGE_SIZE);
- addr = alloc_shstk(size);
+ addr = alloc_shstk(size, 0);

if (IS_ERR((void *)addr))
return PTR_ERR((void *)addr);
@@ -226,7 +231,7 @@ int cet_setup_thread_shstk(struct task_struct *tsk)
if (in_compat_syscall())
size /= 4;
size = round_up(size, PAGE_SIZE);
- addr = alloc_shstk(size);
+ addr = alloc_shstk(size, 0);

if (IS_ERR((void *)addr)) {
cet->shstk_base = 0;
diff --git a/arch/x86/kernel/cet_prctl.c b/arch/x86/kernel/cet_prctl.c
index a8e68fefd524..364ed2420202 100644
--- a/arch/x86/kernel/cet_prctl.c
+++ b/arch/x86/kernel/cet_prctl.c
@@ -35,12 +35,16 @@ static int handle_alloc_shstk(u64 arg2)
unsigned long arg;
unsigned long addr = 0;
unsigned long size = 0;
+ int map_32bit;

if (get_user(arg, (unsigned long __user *)arg2))
return -EFAULT;

+ map_32bit = (arg & ARCH_X86_CET_ALLOC_SHSTK_32BIT) ? 1 : 0;
+ arg &= ~(ARCH_X86_CET_ALLOC_SHSTK_32BIT);
+
size = arg;
- err = cet_alloc_shstk(&arg);
+ err = cet_alloc_shstk(&arg, map_32bit);
if (err)
return err;

--
2.21.0