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

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Message-ID: <ZAnpTYV55gbdROxx@zn.tnic>
Date:   Thu, 9 Mar 2023 15:12:29 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH v7 30/41] x86/shstk: Handle thread shadow stack

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.

https://people.kernel.org/tglx/notes-about-netiquette