[PATCH 1/2] linux-user: Fix siginfo_t contents when jumping to non-readable pages

[Ilya Leoshkevich <iii@linux.ibm.com>]

When the first instruction of a translation block is located in a
non-readable page, qemu-user fills siginfo_t correctly. For the other
instructions the result is as if it were the first instruction, which
is not correct.

The reason is that the current logic expects translate_insn() hook to
stop at the page boundary. This way only the first instruction can
cause a SEGV. However, this is quite difficult to properly implement
when the problematic instruction crosses a page boundary, and indeed
the actual implementations do not do this. Note that this can also
break self-modifying code detection when only bytes on the second page
are modified, but this is outside of the scope of this patch.

Instead of chaning all the translators, do a much simpler thing: when
such a situation is detected, start from scratch and stop right before
the problematic instruction.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 accel/tcg/translate-all.c | 16 +++++++++++-----
 accel/tcg/translator.c    | 25 +++++++++++++++++++++++++
 include/hw/core/cpu.h     |  2 ++
 linux-user/signal.c       |  5 +++++
 4 files changed, 43 insertions(+), 5 deletions(-)

diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
index ef62a199c7..b4766f4661 100644
--- a/accel/tcg/translate-all.c
+++ b/accel/tcg/translate-all.c
@@ -2295,12 +2295,18 @@ void page_set_flags(target_ulong start, target_ulong end, int flags)
          len != 0;
          len -= TARGET_PAGE_SIZE, addr += TARGET_PAGE_SIZE) {
         PageDesc *p = page_find_alloc(addr >> TARGET_PAGE_BITS, 1);
+        bool invalidate;
 
-        /* If the write protection bit is set, then we invalidate
-           the code inside.  */
-        if (!(p->flags & PAGE_WRITE) &&
-            (flags & PAGE_WRITE) &&
-            p->first_tb) {
+        /*
+         * If the write protection bit is set, then we invalidate the code
+         * inside.  For qemu-user, we need to do this when PAGE_READ is cleared
+         * as well, in order to force a SEGV when trying to run this code.
+         */
+        invalidate = !(p->flags & PAGE_WRITE) && (flags & PAGE_WRITE);
+#ifdef CONFIG_USER_ONLY
+        invalidate |= (p->flags & PAGE_READ) && !(flags & PAGE_READ);
+#endif
+        if (invalidate && p->first_tb) {
             tb_invalidate_phys_page(addr, 0);
         }
         if (reset_target_data) {
diff --git a/accel/tcg/translator.c b/accel/tcg/translator.c
index fe7af9b943..e444c17515 100644
--- a/accel/tcg/translator.c
+++ b/accel/tcg/translator.c
@@ -57,6 +57,18 @@ void translator_loop(const TranslatorOps *ops, DisasContextBase *db,
     uint32_t cflags = tb_cflags(tb);
     bool plugin_enabled;
 
+    /*
+     * In case translate_insn hook touched an unreadable page, redo the
+     * translation until the problematic instruction.  We cannot just throw
+     * away the trailing ops, because the hook could have changed DisasContext.
+     */
+    tcg_debug_assert(!cpu->translator_jmp);
+    if (sigsetjmp(cpu->translator_jmp_env, 1) != 0) {
+        cpu->translator_jmp = false;
+        tcg_remove_ops_after(NULL);
+        max_insns = db->num_insns - 1;
+    }
+
     /* Initialize DisasContext */
     db->tb = tb;
     db->pc_first = tb->pc;
@@ -122,8 +134,21 @@ void translator_loop(const TranslatorOps *ops, DisasContextBase *db,
             db->is_jmp = DISAS_TOO_MANY;
             break;
         }
+
+        /*
+         * Propagate SEGVs from the first instruction to the guest and handle
+         * the rest. This way guest's siginfo_t gets accurate pc and si_addr.
+         */
+        cpu->translator_jmp = true;
     }
 
+    /*
+     * Clear translator_jmp on all ways out of this function, otherwise
+     * instructions that fetch code as part of their operation will be
+     * confused.
+     */
+    cpu->translator_jmp = false;
+
     /* Emit code to exit the TB, as indicated by db->is_jmp.  */
     ops->tb_stop(db, cpu);
     gen_tb_end(db->tb, db->num_insns);
diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
index 500503da13..6c1829b7f5 100644
--- a/include/hw/core/cpu.h
+++ b/include/hw/core/cpu.h
@@ -349,6 +349,8 @@ struct CPUState {
     int64_t icount_extra;
     uint64_t random_seed;
     sigjmp_buf jmp_env;
+    bool translator_jmp;
+    sigjmp_buf translator_jmp_env;
 
     QemuMutex work_mutex;
     QSIMPLEQ_HEAD(, qemu_work_item) work_list;
diff --git a/linux-user/signal.c b/linux-user/signal.c
index 8d29bfaa6b..f7e77c8d2e 100644
--- a/linux-user/signal.c
+++ b/linux-user/signal.c
@@ -833,6 +833,11 @@ static void host_signal_handler(int host_sig, siginfo_t *info, void *puc)
         abi_ptr guest_addr;
         bool is_write;
 
+        /* Translator wants to handle this. */
+        if (helper_retaddr == 1 && cpu->translator_jmp) {
+            siglongjmp(cpu->translator_jmp_env, 1);
+        }
+
         host_addr = (uintptr_t)info->si_addr;
 
         /*
-- 
2.35.3