Re: [PATCH v7 30/41] x86/shstk: Handle thread shadow stack

Borislav Petkov
2023-03-09T14:14:11Z

On Wed, Mar 08, 2023 at 08:03:17PM +0000, Edgecombe, Rick P wrote:

Btw,

pls try to trim your replies as I need ot scroll through pages of quoted
text to find the response.

> Sure. Sometimes people tell me to only ifdef out whole functions to
> make it easier to read. I suppose in this case it's not hard to see.

Yeah, the less ifdeffery we have, the better.

> If the default SSP value logic is too hidden, what about some clearer
> code and comments, like this?

The problem with this function is that it needs to return three things:

* success:
** 0
or
** shadow stack address
* failure: due to allocation.

How about this below instead? (totally untested ofc):

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index bf703f53fa49..6e323d4e32fc 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -142,7 +142,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
struct inactive_task_frame *frame;
struct fork_frame *fork_frame;
struct pt_regs *childregs;
- unsigned long shstk_addr = 0;
+ unsigned long shstk_addr;
int ret = 0;

childregs = task_pt_regs(p);
@@ -178,10 +178,9 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
#endif

/* Allocate a new shadow stack for pthread if needed */
- ret = shstk_alloc_thread_stack(p, clone_flags, args->stack_size,
- &shstk_addr);
- if (ret)
- return ret;
+ shstk_addr = shstk_alloc_thread_stack(p, clone_flags, args->stack_size);
+ if (IS_ERR_VALUE(shstk_addr))
+ return PTR_ERR((void *)shstk_addr);

fpu_clone(p, clone_flags, args->fn, shstk_addr);

diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index 13c02747386f..b1668b499e9a 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -157,8 +157,8 @@ void reset_thread_features(void)
current->thread.features_locked = 0;
}

-int shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags,
- unsigned long stack_size, unsigned long *shstk_addr)
+unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags,
+ unsigned long stack_size)
{
struct thread_shstk *shstk = &tsk->thread.shstk;
unsigned long addr, size;
@@ -180,14 +180,12 @@ int shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags,
size = adjust_shstk_size(stack_size);
addr = alloc_shstk(size);
if (IS_ERR_VALUE(addr))
- return PTR_ERR((void *)addr);
+ return addr;

shstk->base = addr;
shstk->size = size;

- *shstk_addr = addr + size;
-
- return 0;
+ return addr + size;
}

static unsigned long get_user_shstk_addr(void)

--
Regards/Gruss,
Boris.