Thread (20 messages) 20 messages, 5 authors, 2021-09-23

Re: [NEEDS-REVIEW] Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

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From: Dave Hansen <hidden>
Date: 2020-09-14 15:06:29
Also in: linux-arch, linux-doc, linux-mm, lkml

Possibly related (same subject, not in this thread)

On 9/11/20 3:59 PM, Yu-cheng Yu wrote:
...
Here are the changes if we take the mprotect(PROT_SHSTK) approach.
Any comments/suggestions?
I still don't like it. :)

I'll also be much happier when there's a proper changelog to accompany
this which also spells out the alternatives any why they suck so much.
quoted hunk ↗ jump to hunk
diff --git a/arch/x86/include/uapi/asm/mman.h b/arch/x86/include/uapi/asm/mman.h
index d4a8d0424bfb..024f006fcfe8 100644
--- a/arch/x86/include/uapi/asm/mman.h
+++ b/arch/x86/include/uapi/asm/mman.h
@@ -4,6 +4,8 @@
 
 #define MAP_32BIT	0x40		/* only give out 32bit addresses */
 
+#define PROT_SHSTK	0x10		/* shadow stack pages */
+
 #ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
 /*
  * Take the 4 protection key bits out of the vma->vm_flags
@@ -19,13 +21,35 @@
 		((vm_flags) & VM_PKEY_BIT2 ? _PAGE_PKEY_BIT2 : 0) |	\
 		((vm_flags) & VM_PKEY_BIT3 ? _PAGE_PKEY_BIT3 : 0))
 
-#define arch_calc_vm_prot_bits(prot, key) (		\
+#define pkey_vm_prot_bits(prot, key) (			\
 		((key) & 0x1 ? VM_PKEY_BIT0 : 0) |      \
 		((key) & 0x2 ? VM_PKEY_BIT1 : 0) |      \
 		((key) & 0x4 ? VM_PKEY_BIT2 : 0) |      \
 		((key) & 0x8 ? VM_PKEY_BIT3 : 0))
+#else
+#define pkey_vm_prot_bits(prot, key)
 #endif
My inner compiler doesn't think this will compile:

	( | shstk_vm_prot_bits(prot))
quoted hunk ↗ jump to hunk
+#define shstk_vm_prot_bits(prot) ( \
+		(static_cpu_has(X86_FEATURE_SHSTK) && (prot & PROT_SHSTK)) ? \
+		VM_SHSTK : 0)
Why do you need to filter PROT_SHSTK twice.  Won't the prot passed in
here be filtered by arch_validate_prot()?
quoted hunk ↗ jump to hunk
+#define arch_calc_vm_prot_bits(prot, key) \
+		(pkey_vm_prot_bits(prot, key) | shstk_vm_prot_bits(prot))
+
IMNHO, this is eminently more readable if you do:

#define arch_calc_vm_prot_bits(prot, key)	\
		(shstk_vm_prot_bits(prot))	\
		  pkey_vm_prot_bits(prot, key))

BTW, can these be static inlines?  I forget if I had a good reason for
making them #defines.
quoted hunk ↗ jump to hunk
+static inline bool arch_validate_prot(unsigned long prot, unsigned long addr)
+{
+	unsigned long supported = PROT_READ | PROT_EXEC | PROT_SEM;
+
+	if (static_cpu_has(X86_FEATURE_SHSTK) && (prot & PROT_SHSTK))
+		supported |= PROT_SHSTK;
+	else
+		supported |= PROT_WRITE;
I generally like to make the common case dirt simple to understand.
That would probably be:

	unsigned long supported = PROT_READ | PROT_WRITE |
				  PROT_EXEC | PROT_SEM;

	if (static_cpu_has(X86_FEATURE_SHSTK) && (prot & PROT_SHSTK)) {
		supported |= PROT_SHSTK;
		// Comment about why SHSTK and WRITE
		// are mutually exclusive.

		supported &= ~PROT_WRITE;
	}
quoted hunk ↗ jump to hunk
 #endif /* _ASM_X86_MMAN_H */
diff --git a/mm/mprotect.c b/mm/mprotect.c
index a8edbcb3af99..520bd8caa005 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -571,6 +571,17 @@ static int do_mprotect_pkey(unsigned long start, size_t
len,
 				goto out;
 		}
 	}
+
+	/*
+	 * Only anonymous mapping is suitable for shadow stack.
+	 */
Why?
quoted hunk ↗ jump to hunk
+	if (prot & PROT_SHSTK) {
+		if (vma->vm_file) {
+			error = -EINVAL;
+			goto out;
+		}
+	}
You can also save a couple of lines there.  The two conditions are
pretty small.
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