[PATCH 0/2] target/arm: Improve user-mode compatibility with JITs

[PATCH 0/2] target/arm: Improve user-mode compatibility with JITs

[John Högberg]

When running in user-mode QEMU currently fails to emulate JITs that
use dual-mapped code to get around W^X restrictions, where one mapping
is writable and one is executable. As it has no way of knowing that a
write to the writable region is reflected in the executable one, it
fails to invalidate previously translated code which leads to a crash
at best.

(Note that system mode is unaffected as the softmmu is fully aware of
what is going on.)

This patch series catches changes to dual-mapped code by honoring the
cache management instructions required to make things work on actual
hardware.

See https://gitlab.com/qemu-project/qemu/-/issues/1034 for more details

John Högberg (2):
  target/arm: Handle IC IVAU to improve compatibility with JITs
  tests/tcg/aarch64: Add testcases for IC IVAU and dual-mapped code

 target/arm/helper.c               |  47 ++++++-
 tests/tcg/aarch64/Makefile.target |   3 +-
 tests/tcg/aarch64/icivau.c        | 204 ++++++++++++++++++++++++++++++
 3 files changed, 250 insertions(+), 4 deletions(-)
 create mode 100644 tests/tcg/aarch64/icivau.c

-- 
2.34.1

[PATCH 1/2] target/arm: Handle IC IVAU to improve compatibility with JITs

[John Högberg]

Unlike architectures with precise self-modifying code semantics
(e.g. x86) ARM processors do not maintain coherency for instruction
execution and memory, and require the explicit use of cache
management instructions as well as an instruction barrier to make
code updates visible (the latter on every core that is going to
execute said code).

While this is required to make JITs work on actual hardware, QEMU
has gotten away with not handling this since it does not emulate
caches, and unconditionally invalidates code whenever the softmmu
or the user-mode page protection logic detects that code has been
modified.

Unfortunately the latter does not work in the face of dual-mapped
code (a common W^X workaround), where one page is executable and
the other is writable: user-mode has no way to connect one with the
other as that is only known to the kernel and the emulated
application.

This commit works around the issue by invalidating code in
IC IVAU instructions.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1034

Co-authored-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: John Högberg <john.hogberg@ericsson.com>
---
 target/arm/helper.c | 47 ++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 44 insertions(+), 3 deletions(-)

diff --git a/target/arm/helper.c b/target/arm/helper.c
index d4bee43bd0..235e3cd0b6 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -5228,6 +5228,36 @@ static void mdcr_el2_write(CPUARMState *env,
const ARMCPRegInfo *ri,
     }
 }
 
+#ifdef CONFIG_USER_ONLY
+/*
+ * `IC IVAU` is handled to improve compatibility with JITs that dual-
map their
+ * code to get around W^X restrictions, where one region is writable
and the
+ * other is executable.
+ *
+ * Since the executable region is never written to we cannot detect
code
+ * changes when running in user mode, and rely on the emulated JIT
telling us
+ * that the code has changed by executing this instruction.
+ */
+static void ic_ivau_write(CPUARMState *env, const ARMCPRegInfo *ri,
+                          uint64_t value)
+{
+    uint64_t icache_line_mask, start_address, end_address;
+    const ARMCPU *cpu;
+
+    cpu = env_archcpu(env);
+
+    icache_line_mask = (4 << extract32(cpu->ctr, 0, 4)) - 1;
+    start_address = value & ~icache_line_mask;
+    end_address = value | icache_line_mask;
+
+    mmap_lock();
+
+    tb_invalidate_phys_range(start_address, end_address);
+
+    mmap_unlock();
+}
+#endif
+
 static const ARMCPRegInfo v8_cp_reginfo[] = {
     /*
      * Minimal set of EL0-visible registers. This will need to be
expanded
@@ -5267,7 +5297,10 @@ static const ARMCPRegInfo v8_cp_reginfo[] = {
     { .name = "CURRENTEL", .state = ARM_CP_STATE_AA64,
       .opc0 = 3, .opc1 = 0, .opc2 = 2, .crn = 4, .crm = 2,
       .access = PL1_R, .type = ARM_CP_CURRENTEL },
-    /* Cache ops: all NOPs since we don't emulate caches */
+    /*
+     * Instruction cache ops. All of these except `IC IVAU` NOP
because we
+     * don't emulate caches.
+     */
     { .name = "IC_IALLUIS", .state = ARM_CP_STATE_AA64,
       .opc0 = 1, .opc1 = 0, .crn = 7, .crm = 1, .opc2 = 0,
       .access = PL1_W, .type = ARM_CP_NOP,
@@ -5280,9 +5313,17 @@ static const ARMCPRegInfo v8_cp_reginfo[] = {
       .accessfn = access_tocu },
     { .name = "IC_IVAU", .state = ARM_CP_STATE_AA64,
       .opc0 = 1, .opc1 = 3, .crn = 7, .crm = 5, .opc2 = 1,
-      .access = PL0_W, .type = ARM_CP_NOP,
+      .access = PL0_W,
       .fgt = FGT_ICIVAU,
-      .accessfn = access_tocu },
+      .accessfn = access_tocu,
+#ifdef CONFIG_USER_ONLY
+      .type = ARM_CP_NO_RAW,
+      .writefn = ic_ivau_write
+#else
+      .type = ARM_CP_NOP
+#endif
+    },
+    /* Cache ops: all NOPs since we don't emulate caches */
     { .name = "DC_IVAC", .state = ARM_CP_STATE_AA64,
       .opc0 = 1, .opc1 = 0, .crn = 7, .crm = 6, .opc2 = 1,
       .access = PL1_W, .accessfn = aa64_cacheop_poc_access,
-- 
2.34.1

[PATCH 2/2] tests/tcg/aarch64: Add testcases for IC IVAU and dual-mapped code

[John Högberg]

https://gitlab.com/qemu-project/qemu/-/issues/1034

Signed-off-by: John Högberg <john.hogberg@ericsson.com>
---
 tests/tcg/aarch64/Makefile.target |   3 +-
 tests/tcg/aarch64/icivau.c        | 204 ++++++++++++++++++++++++++++++
 2 files changed, 206 insertions(+), 1 deletion(-)
 create mode 100644 tests/tcg/aarch64/icivau.c

diff --git a/tests/tcg/aarch64/Makefile.target
b/tests/tcg/aarch64/Makefile.target
index 3430fd3cd8..de6566d0d4 100644
--- a/tests/tcg/aarch64/Makefile.target
+++ b/tests/tcg/aarch64/Makefile.target
@@ -9,9 +9,10 @@ AARCH64_SRC=$(SRC_PATH)/tests/tcg/aarch64
 VPATH 		+= $(AARCH64_SRC)
 
 # Base architecture tests
-AARCH64_TESTS=fcvt pcalign-a64
+AARCH64_TESTS=fcvt pcalign-a64 icivau
 
 fcvt: LDFLAGS+=-lm
+icivau: LDFLAGS+=-lrt
 
 run-fcvt: fcvt
 	$(call run-test,$<,$(QEMU) $<, "$< on $(TARGET_NAME)")
diff --git a/tests/tcg/aarch64/icivau.c b/tests/tcg/aarch64/icivau.c
new file mode 100644
index 0000000000..ff80d3d868
--- /dev/null
+++ b/tests/tcg/aarch64/icivau.c
@@ -0,0 +1,204 @@
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <string.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <fcntl.h>
+
+#define PAYLOAD_SIZE (256)
+
+typedef int (*SelfModTestPtr)(char *, const char*, int);
+typedef int (*CompareTestPtr)(int, int);
+
+void flush_icache(const char *exec_data, size_t length)
+{
+    size_t dcache_stride, icache_stride, i;
+    unsigned long ctr_el0;
+
+    /*
+     * Step according to minimum cache sizes, as the cache maintenance
+     * instructions operate on the cache line of the given address.
+     *
+     * We assume that exec_data is properly aligned.
+     */
+    __asm__("mrs %0, ctr_el0\n" : "=r"(ctr_el0));
+    dcache_stride = (4 << ((ctr_el0 >> 16) & 0xF));
+    icache_stride = (4 << (ctr_el0 & 0xF));
+
+    for (i = 0; i < length; i += dcache_stride) {
+        const char *dc_addr = &exec_data[i];
+        __asm__ ("dc cvau, %x[dc_addr]\n"
+                 : /* no outputs */
+                 : [dc_addr] "r"(dc_addr)
+                 : "memory");
+    }
+
+    __asm__ ("dmb ish\n");
+
+    for (i = 0; i < length; i += icache_stride) {
+        const char *ic_addr = &exec_data[i];
+        __asm__ ("ic ivau, %x[ic_addr]\n"
+                 : /* no outputs */
+                 : [ic_addr] "r"(ic_addr)
+                 : "memory");
+    }
+
+    __asm__ ("dmb ish\n"
+             "isb sy\n");
+}
+
+/*
+ * The unmodified assembly of this function returns 0, it self-
modifies to
+ * return the value indicated by new_move.
+ */
+int self_modification_payload(char *rw_data, const char *exec_data,
+                              int new_move)
+{
+    register int result __asm__ ("w0") = new_move;
+
+    __asm__ (/* Get the writable address of __modify_me. */
+             "sub %x[rw_data], %x[rw_data], %x[exec_data]\n"
+             "adr %x[exec_data], __modify_me\n"
+             "add %x[rw_data], %x[rw_data], %x[exec_data]\n"
+             /* Overwrite the `MOV W0, #0` with the new move. */
+             "str %w[result], [%x[rw_data]]\n"
+             /*
+              * Mark the code as modified.
+              *
+              * Note that we align to the nearest 64 bytes in an
attempt to put
+              * the flush sequence in the same cache line as the
modified move.
+              */
+             ".align 6\n"
+             "dc cvau, %x[exec_data]\n"
+             ".align 2\n"
+             "dmb ish\n"
+             "ic ivau, %x[exec_data]\n"
+             "dmb ish\n"
+             "isb sy\n"
+             "__modify_me: mov w0, #0x0\n"
+             : [result] "+r"(result),
+               [rw_data] "+r"(rw_data),
+               [exec_data] "+r"(exec_data)
+             : /* No untouched inputs */
+             : "memory");
+
+    return result;
+}
+
+int self_modification_test(char *rw_data, const char *exec_data)
+{
+    SelfModTestPtr copied_ptr = (SelfModTestPtr)exec_data;
+    int i;
+
+    /*
+     * Bluntly assumes that the payload is position-independent and
not larger
+     * than PAYLOAD_SIZE.
+     */
+    memcpy(rw_data, self_modification_payload, PAYLOAD_SIZE);
+
+    /*
+     * Notify all PEs that the code at exec_data has been altered.
+     *
+     * For completeness we could assert that we should fail when this
is
+     * omitted, which works in user mode and on actual hardware as the
+     * modification won't "take," but doesn't work in system mode as
the
+     * softmmu handles everything for us.
+     */
+    flush_icache(exec_data, PAYLOAD_SIZE);
+
+    for (i = 1; i < 10; i++) {
+        const int mov_w0_template = 0x52800000;
+
+        /* MOV W0, i */
+        if (copied_ptr(rw_data, exec_data, mov_w0_template | (i << 5))
!= i) {
+            return 0;
+        }
+    }
+
+    return 1;
+}
+
+int compare_copied(char *rw_data, const char *exec_data,
+                   int (*reference_ptr)(int, int))
+{
+    CompareTestPtr copied_ptr = (CompareTestPtr)exec_data;
+    int a, b;
+
+    memcpy(rw_data, reference_ptr, PAYLOAD_SIZE);
+    flush_icache(exec_data, PAYLOAD_SIZE);
+
+    for (a = 1; a < 10; a++) {
+        for (b = 1; b < 10; b++) {
+            if (copied_ptr(a, b) != reference_ptr(a, b)) {
+                return 0;
+            }
+        }
+    }
+
+    return 1;
+}
+
+int compare_alpha(int a, int b)
+{
+    return a + b;
+}
+
+int compare_beta(int a, int b)
+{
+    return a - b;
+}
+
+int compare_gamma(int a, int b)
+{
+    return a * b;
+}
+
+int compare_delta(int a, int b)
+{
+    return a / b;
+}
+
+int main(int argc, char **argv)
+{
+    const char *shm_name = "qemu-test-tcg-aarch64-icivau";
+    int fd;
+
+    fd = shm_open(shm_name, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
+
+    if (fd < 0) {
+        return EXIT_FAILURE;
+    }
+
+    /* Unlink early to avoid leaving garbage in case the test crashes.
*/
+    shm_unlink(shm_name);
+
+    if (ftruncate(fd, PAYLOAD_SIZE) == 0) {
+        const char *exec_data;
+        char *rw_data;
+
+        rw_data = mmap(0, PAYLOAD_SIZE, PROT_READ | PROT_WRITE,
MAP_SHARED,
+                       fd, 0);
+        exec_data = mmap(0, PAYLOAD_SIZE, PROT_READ | PROT_EXEC,
MAP_SHARED,
+                         fd, 0);
+
+        if (rw_data && exec_data) {
+            CompareTestPtr compare_tests[4] = {compare_alpha,
+                                               compare_beta,
+                                               compare_gamma,
+                                               compare_delta};
+            int success, i;
+
+            success = self_modification_test(rw_data, exec_data);
+
+            for (i = 0; i < 4; i++) {
+                success &= compare_copied(rw_data, exec_data,
compare_tests[i]);
+            }
+
+            if (success) {
+                return EXIT_SUCCESS;
+            }
+        }
+    }
+
+    return EXIT_FAILURE;
+}
-- 
2.34.1

Re: [PATCH 1/2] target/arm: Handle IC IVAU to improve compatibility with JITs

[Michael Tokarev]

11.06.2023 14:53, John Högberg wrote:
...
>   
> +#ifdef CONFIG_USER_ONLY
> +/*
> + * `IC IVAU` is handled to improve compatibility with JITs that dual-
> map their
> + * code to get around W^X restrictions, where one region is writable
> and the
> + * other is executable.

The patches seems to be line-wrap-damaged, and also base64-encoded.
Dunno if it's possible to un-damage it easily...

/mjt

Re: [PATCH 1/2] target/arm: Handle IC IVAU to improve compatibility with JITs

[John Högberg]

Sorry, something went sideways on my end. I'll try re-sending the
patchset tomorrow through the web-based interface mentioned in the
documentation, hopefully that will be foolproof. :)

Regards,
John Högberg

-----Original Message-----
From: Michael Tokarev <mjt@tls.msk.ru>
To: John Högberg <john.hogberg@ericsson.com>, qemu-devel@nongnu.org <qemu-devel@nongnu.org>
Cc: peter.maydell@linaro.org <peter.maydell@linaro.org>
Subject: Re: [PATCH 1/2] target/arm: Handle IC IVAU to improve compatibility with JITs
Date: Sun, 11 Jun 2023 19:47:58 +0300

11.06.2023 14:53, John Högberg wrote:
...
>   
> +#ifdef CONFIG_USER_ONLY
> +/*
> + * `IC IVAU` is handled to improve compatibility with JITs that dual-
> map their
> + * code to get around W^X restrictions, where one region is writable
> and the
> + * other is executable.

The patches seems to be line-wrap-damaged, and also base64-encoded.
Dunno if it's possible to un-damage it easily...

/mjt