From mboxrd@z Thu Jan 1 00:00:00 1970 From: linux@armlinux.org.uk (Russell King - ARM Linux) Date: Thu, 6 Sep 2018 13:48:11 +0100 Subject: [RESEND PATCH 3/8] ARM: spectre-v1,v1.1: provide helpers for address sanitization In-Reply-To: <1535447316-32187-4-git-send-email-julien.thierry@arm.com> References: <1535447316-32187-1-git-send-email-julien.thierry@arm.com> <1535447316-32187-4-git-send-email-julien.thierry@arm.com> Message-ID: <20180906124810.GO30658@n2100.armlinux.org.uk> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Hi Julien, On Tue, Aug 28, 2018 at 10:08:31AM +0100, Julien Thierry wrote: > + .macro uaccess_mask_range_ptr, addr:req, size:req, limit:req, tmp:req > +#ifdef CONFIG_CPU_SPECTRE > + sub \tmp, \limit, #1 > + subs \tmp, \tmp, \addr @ tmp = limit - 1 - addr > + subhss \tmp, \tmp, \size @ if (tmp >= 0) tmp = limit - 1 - (addr + size) > + movlo \addr, #0 @ if (tmp < 0) addr = NULL > + csdb I'be been thinking about this and my original code. This seems to suffer from a problem in that the last access below the address limit becomes unaccessible. Let's take an example. If limit is 0xbf000000, the 32-bit word at 0xbefffffc should be accessible, so addr=0xbefffffc and size=4. sub \tmp, \limit, #1 tmp = 0xbeffffff subs \tmp, \tmp, \addr tmp = 0xbeffffff - 0xbefffffc = 3 subhss \tmp, \tmp, \size tmp = 3 - 4 = -1 which means we zero the pointer. This is obviously incorrect behaviour, because this word should obviously be accessible - it is entirely below the limit. I should point out that the existing code seems to suffer from the same issue: @ r1=addr, r2=size r3=limit adds ip, r1, r2 ip=0xbf000000 C=0 sub r3, r3, #1 r3=0xbeffffff cmpcc ip, r3 C=ip >= r3 An obvious solution to your code would be to add the '1' back in, but realising that fails when addr == 0 and limit=0. sub \tmp, \limit, #1 tmp = 0xffffffff subs \tmp, \tmp, \addr tmp = 0xffffffff - 0 = 0xffffffff add \tmp, \tmp, #1 tmp = 0 subhss \tmp, \tmp, \size tmp = 0 - 4 = -4 However, that doesn't matter as page 0 should never be mapped, and in any case, if addr was zero and we then make it zero again, we haven't changed anything. > +#endif > + .endm > + > .macro uaccess_disable, tmp, isb=1 > #ifdef CONFIG_CPU_SW_DOMAIN_PAN > /* > diff --git a/arch/arm/include/asm/uaccess.h b/arch/arm/include/asm/uaccess.h > index 2e18b91..9462b8b 100644 > --- a/arch/arm/include/asm/uaccess.h > +++ b/arch/arm/include/asm/uaccess.h > @@ -100,6 +100,31 @@ static inline void set_fs(mm_segment_t fs) > __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL)) > > /* > + * Sanitise a uaccess pointer such that it becomes NULL if addr+size > + * is above the current addr_limit. > + */ > +#define uaccess_mask_range_ptr(ptr, size) \ > + ((__typeof__(ptr))__uaccess_mask_range_ptr(ptr, size)) > +static inline void __user *__uaccess_mask_range_ptr(const void __user *ptr, > + size_t size) > +{ > + void __user *safe_ptr = (void __user *)ptr; > + unsigned long tmp; > + > + asm volatile( > + " sub %1, %3, #1\n" > + " subs %1, %1, %0\n" > + " subhss %1, %1, %2\n" > + " movlo %0, #0\n" > + : "+r" (safe_ptr), "=&r" (tmp) > + : "r" (size), "r" (current_thread_info()->addr_limit) > + : "cc"); > + > + csdb(); > + return safe_ptr; > +} > + > +/* > * Single-value transfer routines. They automatically use the right > * size if we just have the right pointer type. Note that the functions > * which read from user space (*get_*) need to take care not to leak > diff --git a/arch/arm/lib/copy_from_user.S b/arch/arm/lib/copy_from_user.S > index a826df3..6709a8d 100644 > --- a/arch/arm/lib/copy_from_user.S > +++ b/arch/arm/lib/copy_from_user.S > @@ -93,11 +93,7 @@ ENTRY(arm_copy_from_user) > #ifdef CONFIG_CPU_SPECTRE > get_thread_info r3 > ldr r3, [r3, #TI_ADDR_LIMIT] > - adds ip, r1, r2 @ ip=addr+size > - sub r3, r3, #1 @ addr_limit - 1 > - cmpcc ip, r3 @ if (addr+size > addr_limit - 1) > - movcs r1, #0 @ addr = NULL > - csdb > + uaccess_mask_range_ptr r1, r2, r3, ip > #endif > > #include "copy_template.S" > -- > 1.9.1 > -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 13.8Mbps down 630kbps up According to speedtest.net: 13Mbps down 490kbps up