• Ralf Baechle's avatar
    MIPS: Fix possible corruption of cache mode by mprotect. · 6d037de9
    Ralf Baechle authored
    The following testcase may result in a page table entries with a invalid
    CCA field being generated:
    
    static void *bindstack;
    
    static int sysrqfd;
    
    static void protect_low(int protect)
    {
    	mprotect(bindstack, BINDSTACK_SIZE, protect);
    }
    
    static void sigbus_handler(int signal, siginfo_t * info, void *context)
    {
    	void *addr = info->si_addr;
    
    	write(sysrqfd, "x", 1);
    
    	printf("sigbus, fault address %p (should not happen, but might)\n",
    	       addr);
    	abort();
    }
    
    static void run_bind_test(void)
    {
    	unsigned int *p = bindstack;
    
    	p[0] = 0xf001f001;
    
    	write(sysrqfd, "x", 1);
    
    	/* Set trap on access to p[0] */
    	protect_low(PROT_NONE);
    
    	write(sysrqfd, "x", 1);
    
    	/* Clear trap on access to p[0] */
    	protect_low(PROT_READ | PROT_WRITE | PROT_EXEC);
    
    	write(sysrqfd, "x", 1);
    
    	/* Check the contents of p[0] */
    	if (p[0] != 0xf001f001) {
    		write(sysrqfd, "x", 1);
    
    		/* Reached, but shouldn't be */
    		printf("badness, shouldn't happen but does\n");
    		abort();
    	}
    }
    
    int main(void)
    {
    	struct sigaction sa;
    
    	sysrqfd = open("/proc/sysrq-trigger", O_WRONLY);
    
    	if (sigprocmask(SIG_BLOCK, NULL, &sa.sa_mask)) {
    		perror("sigprocmask");
    		return 0;
    	}
    
    	sa.sa_sigaction = sigbus_handler;
    	sa.sa_flags = SA_SIGINFO | SA_NODEFER | SA_RESTART;
    	if (sigaction(SIGBUS, &sa, NULL)) {
    		perror("sigaction");
    		return 0;
    	}
    
    	bindstack = mmap(NULL,
    			 BINDSTACK_SIZE,
    			 PROT_READ | PROT_WRITE | PROT_EXEC,
    			 MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
    	if (bindstack == MAP_FAILED) {
    		perror("mmap bindstack");
    		return 0;
    	}
    
    	printf("bindstack: %p\n", bindstack);
    
    	run_bind_test();
    
    	printf("done\n");
    
    	return 0;
    }
    
    There are multiple ingredients for this:
    
     1) PAGE_NONE is defined to _CACHE_CACHABLE_NONCOHERENT, which is CCA 3
        on all platforms except SB1 where it's CCA 5.
     2) _page_cachable_default must have bits set which are not set
        _CACHE_CACHABLE_NONCOHERENT.
     3) Either the defective version of pte_modify for XPA or the standard
        version must be in used.  However pte_modify for the 36 bit address
        space support is no affected.
    
    In that case additional bits in the final CCA mode may generate an invalid
    value for the CCA field.  On the R10000 system where this was tracked
    down for example a CCA 7 has been observed, which is Uncached Accelerated.
    
    Fixed by:
    
     1) Using the proper CCA mode for PAGE_NONE just like for all the other
        PAGE_* pte/pmd bits.
     2) Fix the two affected variants of pte_modify.
    
    Further code inspection also shows the same issue to exist in pmd_modify
    which would affect huge page systems.
    
    Issue in pte_modify tracked down by Alastair Bridgewater, PAGE_NONE
    and pmd_modify issue found by me.
    
    The history of this goes back beyond Linus' git history.  Chris Dearman's
    commit 35133692 ("[MIPS] Allow setting of
    the cache attribute at run time.") missed the opportunity to fix this
    but it was originally introduced in lmo commit
    d523832cf12007b3242e50bb77d0c9e63e0b6518 ("Missing from last commit.")
    and 32cc38229ac7538f2346918a09e75413e8861f87 ("New configuration option
    CONFIG_MIPS_UNCACHED.")
    Signed-off-by: default avatarRalf Baechle <ralf@linux-mips.org>
    Reported-by: default avatarAlastair Bridgewater <alastair.bridgewater@gmail.com>
    6d037de9
pgtable.h 17.5 KB