[BUG] arm64: an infinite loop in generic_perform_write()

[Catalin Marinas]

On Thu, Jun 24, 2021 at 04:27:17PM +0000, Al Viro wrote:
> On Thu, Jun 24, 2021 at 02:22:27PM +0100, Robin Murphy wrote:
> > FWIW I think the only way to make the kernel behaviour any more robust here
> > would be to make the whole uaccess API more expressive, such that rather
> > than simply saying "I only got this far" it could actually differentiate
> > between stopping due to a fault which may be recoverable and worth retrying,
> > and one which definitely isn't.
> 
> ... and propagate that "more expressive" information through what, 3 or 4
> levels in the call chain?  
> 
> From include/linux/uaccess.h:
> 
>  * If raw_copy_{to,from}_user(to, from, size) returns N, size - N bytes starting
>  * at to must become equal to the bytes fetched from the corresponding area
>  * starting at from.  All data past to + size - N must be left unmodified.
>  *
>  * If copying succeeds, the return value must be 0.  If some data cannot be
>  * fetched, it is permitted to copy less than had been fetched; the only
>  * hard requirement is that not storing anything at all (i.e. returning size)
>  * should happen only when nothing could be copied.  In other words, you don't
>  * have to squeeze as much as possible - it is allowed, but not necessary.
> 
> arm64 instances violate the aforementioned hard requirement.

After reading the above a few more times, I think I get it. The key
sentence is: not storing anything at all should happen only when nothing
could be copied. In the MTE case, something can still be copied.

> Please, fix
> it there; it's not hard.  All you need is an exception handler in .Ltiny15
> that would fall back to (short) byte-by-byte copy if the faulting address
> happened to be unaligned.  Or just do one-byte copy, not that it had been
> considerably cheaper than a loop.  Will be cheaper than propagating that extra
> information up the call chain, let alone paying for extra ->write_begin()
> and ->write_end() for single byte in generic_perform_write().

Yeah, it's definitely fixable in the arch code. I misread the above
requirements and thought it could be fixed in the core code.

Quick hack, though I think in the actual exception handling path in .S
more sense (and it needs the copy_to_user for symmetry):

diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
index b5f08621fa29..903f8a2a457b 100644
--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h
@@ -415,6 +415,15 @@ extern unsigned long __must_check __arch_copy_from_user(void *to, const void __u
 	uaccess_ttbr0_enable();						\
 	__acfu_ret = __arch_copy_from_user((to),			\
 				      __uaccess_mask_ptr(from), (n));	\
+	if (__acfu_ret == n) {						\
+		int __cfu_err = 0;					\
+		char __cfu_val;						\
+		__raw_get_mem("ldtr", __cfu_val, (char *)from, __cfu_err);\
+		if (!__cfu_err) {					\
+			*(char *)to = __cfu_val;			\
+			__acfu_ret--;					\
+		}							\
+	}								\
 	uaccess_ttbr0_disable();					\
 	__acfu_ret;							\
 })

Of course, it only fixes the MTE problem, I'll ignore the MMIO case
(though it may work in certain configurations like synchronous faults).

-- 
Catalin