Re: [PATCH] arm64: mm: disable PAN during caches_clean_inval_user_pou

[Mark Rutland]

On Mon, Jan 08, 2024 at 04:37:37PM +0000, Brandt, Oliver - Lenze wrote:
> > On Mon, Jan 08, 2024 at 01:00:39PM +0000, Brandt, Oliver - Lenze wrote:
> > > Using the cacheflush() syscall from an 32-bit user-space fails when
> > > ARM64_PAN is used. We 'll get an endless loop:
> > > 
> > >       1. executing "dc cvau, x2" results in raising an abort
> > >       2. abort handler does not fix the reason for the abort and
> > >          returns to 1.
> > > 
> > > Disabling PAN for the time of the cache maintenance fixes this.
> > 
> > Hmm... the ARM ARM says PSTATE.PAN is not supposed to affect DC CVAU.
> > 
> > Looking at the latest ARM ARM (ARM DDI 0487J.a), R_PMTWB states:
> > 
> > > The PSTATE.PAN bit has no effect on all of the following:
> > > 
> > > o Instruction fetches.
> > > o Data cache instructions, except DC ZVA.
> > > o If FEAT_PAN2 is not implemented, then address translation instructions.
> > > o If FEAT_PAN2 is implemented, then the address translation instructions
> > >   other than AT S1E1RP and AT S1E1WP.
> > 
> > So IIUC, DC CVAU shouldn't be affected by PAN.
> 
> Ups... Sorry, didn't noticed this.

No worries; this is not at all obvious!

> > This could be a CPU bug; which CPU are you seeing this with?
> 
> I've stumbled about this while using Intel's simulator "Simics" with a
> model of the upcoming "Agilex5 socfpga". The "Agilex5" is a SoC
> containing two Cortex A76 and two Cortex A55.

Ah, so it could be a bug in Simics, then.

> We are expecting the real silicon in a couple of weeks. Seems to be a
> good idea to check the silicon first. Sorry to bother you with this.

Just to make sure I ran a quick test on an AML-905D3-CC board (quad-core
Cortex-A55), and AFAICT we're not taking unexpected faults. Log below,
including the test case.

If you do see problems on silicon, please let us know!

Mark.

---->8----
mark@flodeboller:~/test/aarch32-cacheflush$ sudo dmesg | grep -i access
[    0.010476] CPU features: detected: Privileged Access Never
mark@flodeboller:~/test/aarch32-cacheflush$ cat test.c
#include <stdio.h>

void cacheflush(void *start, size_t size)
{
        printf("Attempting flush of [%p..%p]\n", start, start + size);
        __builtin___clear_cache(start, start + size);
}

int main(int argc, char *argv[])
{
        static char buf[4096];

        cacheflush(buf, sizeof(buf));

        cacheflush(NULL, sizeof(buf));

        return 0;
}
mark@flodeboller:~/test/aarch32-cacheflush$ arm-linux-gnueabihf-gcc test.c -o test -O3
mark@flodeboller:~/test/aarch32-cacheflush$ file test
test: ELF 32-bit LSB pie executable, ARM, EABI5 version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-armhf.so.3, BuildID[sha1]=a53713f6b623b9b7c29cee4dc615fb7d43a0dcb6, for GNU/Linux 3.2.0, not stripped
mark@flodeboller:~/test/aarch32-cacheflush$ strace ./test
execve("./test", ["./test"], 0xffffd7e09890 /* 25 vars */ <unfinished ...>
[ Process PID=7682 runs in 32 bit mode. ]
strace: WARNING: Proper structure decoding for this personality is not supported, please consider building strace with mpers support enabled.
<... execve resumed>)                   = 0
brk(NULL)                               = 0x222b000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
statx(3, "", AT_STATX_SYNC_AS_STAT|AT_NO_AUTOMOUNT|AT_EMPTY_PATH, STATX_BASIC_STATS, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0644, stx_size=31402, ...}) = 0
mmap2(NULL, 31402, PROT_READ, MAP_PRIVATE, 3, 0) = 0xf7b0b000
close(3)                                = 0
openat(AT_FDCWD, "/lib/arm-linux-gnueabihf/libc.so.6", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
read(3, "\177ELF\1\1\1\3\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0i\344\1\0004\0\0\0"..., 512) = 512
statx(3, "", AT_STATX_SYNC_AS_STAT|AT_NO_AUTOMOUNT|AT_EMPTY_PATH, STATX_BASIC_STATS, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0755, stx_size=1102644, ...}) = 0
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xf7b09000
mmap2(NULL, 1139660, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xf79f2000
mmap2(0xf7afc000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x109000) = 0xf7afc000
mmap2(0xf7aff000, 37836, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xf7aff000
close(3)                                = 0
set_tls(0xf7b09ce0)                     = 0
set_tid_address(0xf7b09848)             = 7682
set_robust_list(0xf7b0984c, 12)         = 0
rseq(0xf7b09cc0, 0x20, 0, 0xe7f5def3)   = 0
mprotect(0xf7afc000, 8192, PROT_READ)   = 0
mprotect(0x572000, 4096, PROT_READ)     = 0
mprotect(0xf7b31000, 4096, PROT_READ)   = 0
ugetrlimit(RLIMIT_STACK, {rlim_cur=8192*1024, rlim_max=RLIM_INFINITY}) = 0
munmap(0xf7b0b000, 31402)               = 0
statx(1, "", AT_STATX_SYNC_AS_STAT|AT_NO_AUTOMOUNT|AT_EMPTY_PATH, STATX_BASIC_STATS, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFCHR|0620, stx_size=0, ...}) = 0
getrandom("\x79\xe4\xe7\x57", 4, GRND_NONBLOCK) = 4
brk(NULL)                               = 0x222b000
brk(0x224c000)                          = 0x224c000
write(1, "Attempting flush of [0x573040..0"..., 41Attempting flush of [0x573040..0x574040]
) = 41
cacheflush(0x573040, 0x574040, 0)       = 0
write(1, "Attempting flush of [(nil)..0x10"..., 36Attempting flush of [(nil)..0x1000]
) = 36
cacheflush(0, 0x1000, 0)                = -1 EFAULT (Bad address)
exit_group(0)                           = ?
+++ exited with 0 +++