Re: [PATCH v0.9.1 2/6] mm, x86/uaccess: add userspace atomic helpers

Peter Zijlstra
2021-11-24T14:46:45Z

On Mon, Nov 22, 2021 at 01:13:23PM -0800, Peter Oskolkov wrote:

> +static inline int __try_cmpxchg_user_32(u32 *uval, u32 __user *uaddr,
> + u32 oldval, u32 newval)

That's a 'try_cmpxchg' function but *NOT* the try_cmpxchg semantics,
please don't do that, fixed in the below.

> +int cmpxchg_user_64(u64 __user *uaddr, u64 *curr_val, u64 new_val)
> +{
> + int ret = -EFAULT;
> + u64 __old = *curr_val;
> +
> + /* Validate proper alignment. */
> + if (unlikely(((unsigned long)uaddr % sizeof(*uaddr)) ||
> + ((unsigned long)curr_val % sizeof(*curr_val))))
> + return -EINVAL;
> +
> + if (unlikely(!access_ok(uaddr, sizeof(*uaddr))))
> + return -EFAULT;
> +
> + pagefault_disable();
> +
> + while (true) {
> + ret = -EFAULT;
> + if (!user_access_begin(uaddr, sizeof(*uaddr)))
> + break;
> +
> + ret = __try_cmpxchg_user_64(curr_val, uaddr, __old, new_val);
> + user_access_end();
> +
> + if (!ret) {
> + ret = *curr_val == __old ? 0 : -EAGAIN;
> + break;
> + }
> +
> + if (fix_pagefault((unsigned long)uaddr, true, sizeof(*uaddr)) < 0)
> + break;
> + }
> +
> + pagefault_enable();
> + return ret;
> +}

Please, just read what you wrote. This scored *really* high on the
WTF'o'meter.

That is aside of:

- that user_access_begin() includes access_ok().
- the fact that having SMAP *inside* a cmpxchg loop is ridiculous.
- that you write cmpxchg inside a loop, but it isn't actually a cmpxchg-loop.

No the real problem is:

- you *DISABLE* pagefaults
- you force the exception handler
- you manually fix up the fault

while you could've just done the op and let the fault handler do it's
thing, that whole function is pointless.

So as a penance for not having looked at this before I wrote you the
replacement. The asm-goto-output variant isn't actually compile tested,
but the old complicated thing is. Also, I'm >.< close to merging the
series that kills .fixup for x86, but the fixup (pun intended) should be
trivial.

Usage can be gleaned from the bigger patch I send you in reply to 0/ but
TL;DR:

if (!user_access_begin(uptr, sizeof(u64)))
return -EFAULT;

unsafe_get_user(old, uptr, Efault);
do {
new = func(old);
} while (!__try_cmpxchg_user(uptr, &old, new, Efault));

user_access_end();

return 0;

Efault:
user_access_end();
return -EFAULT;

Then if called within pagefault_disable(), it'll get -EFAULT more, if
called without it, it'll just take the fault and try to fix it up if at
all possible.

---
diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 33a68407def3..909c48083c4f 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -341,6 +341,37 @@ do { \
: [umem] "m" (__m(addr)) \
: : label)

+#define __try_cmpxchg_user_asm(itype, _ptr, _pold, _new, label) ({ \
+ bool success; \
+ __typeof__(_ptr) _old = (__typeof__(_ptr))(_pold); \
+ __typeof__(*(_ptr)) __old = *_old; \
+ __typeof__(*(_ptr)) __new = (_new); \
+ asm_volatile_goto("\n" \
+ "1: " LOCK_PREFIX "cmpxchg"itype" %[new], %[ptr]\n"\
+ _ASM_EXTABLE_UA(1b, %l[label]) \
+ : CC_OUT(z) (success), \
+ [ptr] "+m" (*_ptr), \
+ [old] "+a" (__old) \
+ : [new] "r" (__new) \
+ : "memory", "cc" \
+ : label); \
+ if (unlikely(!success)) \
+ *_old = __old; \
+ likely(success); })
+
+
+#define __xchg_user_asm(itype, _ptr, _val, label) ({ \
+ __typeof__(*(_ptr)) __ret = (_val); \
+ asm_volatile_goto("\n" \
+ "1: " LOCK_PREFIX "xchg"itype" %[var], %[ptr]\n"\
+ _ASM_EXTABLE_UA(1b, %l[label]) \
+ : [var] "+r" (__ret). \
+ [ptr] "+m" (*(_ptr)) \
+ : \
+ : "memory", "cc" \
+ : label); \
+ __ret; })
+
#else // !CONFIG_CC_HAS_ASM_GOTO_OUTPUT

#ifdef CONFIG_X86_32
@@ -411,8 +442,83 @@ do { \
: [umem] "m" (__m(addr)), \
[efault] "i" (-EFAULT), "0" (err))

+#define __try_cmpxchg_user_asm(itype, _ptr, _pold, _new, label) ({ \
+ int __err = 0; \
+ bool success; \
+ __typeof__(_ptr) _old = (__typeof__(_ptr))(_pold); \
+ __typeof__(*(_ptr)) __old = *_old; \
+ __typeof__(*(_ptr)) __new = (_new); \
+ asm volatile("\n" \
+ "1: " LOCK_PREFIX "cmpxchg"itype" %[new], %[ptr]\n"\
+ CC_SET(z) \
+ "2:\n" \
+ ".pushsection .fixup,\"ax\"\n" \
+ "3: mov %[efault], %[errout]\n" \
+ " jmp 2b\n" \
+ ".popsection\n" \
+ _ASM_EXTABLE_UA(1b, 3b) \
+ : CC_OUT(z) (success), \
+ [errout] "+r" (__err), \
+ [ptr] "+m" (*_ptr), \
+ [old] "+a" (__old) \
+ : [new] "r" (__new), \
+ [efault] "i" (-EFAULT) \
+ : "memory", "cc"); \
+ if (unlikely(__err)) \
+ goto label; \
+ if (unlikely(!success)) \
+ *_old = __old; \
+ likely(success); })
+
+#define __xchg_user_asm(itype, _ptr, _val, label) ({ \
+ int __err = 0; \
+ __typeof__(*(_ptr)) __ret = (_val); \
+ asm volatile("\n" \
+ "1: " LOCK_PREFIX "xchg"itype" %[var], %[ptr]\n" \
+ "2:\n" \
+ ".pushsection .fixup,\"ax\"\n" \
+ "3: mov %[efault], %[errout]\n" \
+ " jmp 2b\n" \
+ ".popsection\n" \
+ _ASM_EXTABLE_UA(1b, 3b) \
+ : [ptr] "+m" (*(_ptr)), \
+ [var] "+r" (__ret), \
+ [errout] "+r" (__err) \
+ : [efault] "i" (-EFAULT) \
+ : "memory", "cc"); \
+ if (unlikely(__err)) \
+ goto label; \
+ __ret; })
+
#endif // CONFIG_CC_HAS_ASM_GOTO_OUTPUT

+extern void __try_cmpxchg_user_wrong_size(void);
+extern void __xchg_user_wrong_size(void);
+
+#define __try_cmpxchg_user(_ptr, _oldp, _nval, _label) ({ \
+ __typeof__(*(_ptr)) __ret; \
+ switch (sizeof(__ret)) { \
+ case 4: __ret = __try_cmpxchg_user_asm("l", (_ptr), (_oldp), \
+ (_nval), _label); \
+ break; \
+ case 8: __ret = __try_cmpxchg_user_asm("q", (_ptr), (_oldp), \
+ (_nval), _label); \
+ break; \
+ default: __try_cmpxchg_user_wrong_size(); \
+ } \
+ __ret; })
+
+#define __xchg_user(_ptr, _nval, _label) ({ \
+ __typeof__(*(_ptr)) __ret; \
+ switch (sizeof(__ret)) { \
+ case 4: __ret = __xchg_user_asm("l", (_ptr), (_nval), _label); \
+ break; \
+ case 8: __ret = __xchg_user_asm("q", (_ptr), (_nval), _label); \
+ break; \
+ default: __xchg_user_wrong_size(); \
+ } \
+ __ret; })
+
/* FIXME: this hack is definitely wrong -AK */
struct __large_struct { unsigned long buf[100]; };
#define __m(x) (*(struct __large_struct __user *)(x))