[Qemu-devel] [PATCH 2.2 v4] linux-user: Fix up timer id handling

[Qemu-devel] [PATCH 2.2 v4] linux-user: Fix up timer id handling

[Alexander Graf]

When creating a timer handle, we give the timer id a special magic offset
of 0xcafe0000. However, we never mask that offset out of the timer id before
we start using it to dereference our timer array. So we always end up aborting
timer operations because the timer id is out of bounds.

This was not an issue before my patch e52a99f756e ("linux-user: Simplify
timerid checks on g_posix_timers range") because before we would blindly mask
anything above the first 16 bits.

This patch simplifies the code around timer id creation by introducing a proper
target_timer_id typedef that is s32, just like Linux has it. It also changes the
magic offset to a value that makes all timer ids be positive.

Reported-by: Tom Musta <tommusta@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>

---

v1 -> v2:

  - Abort when magic is missing

v2 -> v3:

  - Squash into a single patch
  - Change magic to always have positive IDs

v3 -> v4:

  - Also handle timer_getoverrun and timer_delete
  - Move timer boundary checks into separate function
---
 linux-user/syscall.c      | 52 +++++++++++++++++++++++++++++++----------------
 linux-user/syscall_defs.h |  5 +----
 2 files changed, 36 insertions(+), 21 deletions(-)

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index a175cc1..cfc4a47 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -5473,6 +5473,25 @@ static int do_openat(void *cpu_env, int dirfd, const char *pathname, int flags,
     return get_errno(sys_openat(dirfd, path(pathname), flags, mode));
 }
 
+#define TIMER_MAGIC 0x0caf0000
+#define TIMER_MAGIC_MASK 0xffff0000
+
+/* Convert QEMU provided timer ID back to internal 16bit index format */
+static target_timer_t get_timer_id(abi_long arg)
+{
+    target_timer_t timerid = arg;
+
+    if ((timerid & TIMER_MAGIC_MASK) != TIMER_MAGIC) {
+        return -TARGET_EINVAL;
+    }
+
+    if (timerid >= ARRAY_SIZE(g_posix_timers)) {
+        return -TARGET_EINVAL;
+    }
+
+    return timerid & 0xffff;
+}
+
 /* do_syscall() should always have a single exit point at the end so
    that actions, such as logging of syscall results, can be performed.
    All errnos that do_syscall() returns must be -TARGET_<errcode>. */
@@ -9579,7 +9598,6 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
         /* args: clockid_t clockid, struct sigevent *sevp, timer_t *timerid */
 
         struct sigevent host_sevp = { {0}, }, *phost_sevp = NULL;
-        struct target_timer_t *ptarget_timer;
 
         int clkid = arg1;
         int timer_index = next_free_host_timer();
@@ -9601,11 +9619,9 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
             if (ret) {
                 phtimer = NULL;
             } else {
-                if (!lock_user_struct(VERIFY_WRITE, ptarget_timer, arg3, 1)) {
+                if (put_user(TIMER_MAGIC | timer_index, arg3, target_timer_t)) {
                     goto efault;
                 }
-                ptarget_timer->ptr = tswap32(0xcafe0000 | timer_index);
-                unlock_user_struct(ptarget_timer, arg3, 1);
             }
         }
         break;
@@ -9617,9 +9633,11 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
     {
         /* args: timer_t timerid, int flags, const struct itimerspec *new_value,
          * struct itimerspec * old_value */
-        target_ulong timerid = arg1;
+        target_timer_t timerid = get_timer_id(arg1);
 
-        if (arg3 == 0 || timerid >= ARRAY_SIZE(g_posix_timers)) {
+        if (timerid < 0) {
+            ret = timerid;
+        } else if (arg3 == 0) {
             ret = -TARGET_EINVAL;
         } else {
             timer_t htimer = g_posix_timers[timerid];
@@ -9638,12 +9656,12 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
     case TARGET_NR_timer_gettime:
     {
         /* args: timer_t timerid, struct itimerspec *curr_value */
-        target_ulong timerid = arg1;
+        target_timer_t timerid = get_timer_id(arg1);
 
-        if (!arg2) {
-            return -TARGET_EFAULT;
-        } else if (timerid >= ARRAY_SIZE(g_posix_timers)) {
-            ret = -TARGET_EINVAL;
+        if (timerid < 0) {
+            ret = timerid;
+        } else if (!arg2) {
+            ret = -TARGET_EFAULT;
         } else {
             timer_t htimer = g_posix_timers[timerid];
             struct itimerspec hspec;
@@ -9661,10 +9679,10 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
     case TARGET_NR_timer_getoverrun:
     {
         /* args: timer_t timerid */
-        target_ulong timerid = arg1;
+        target_timer_t timerid = get_timer_id(arg1);
 
-        if (timerid >= ARRAY_SIZE(g_posix_timers)) {
-            ret = -TARGET_EINVAL;
+        if (timerid < 0) {
+            ret = timerid;
         } else {
             timer_t htimer = g_posix_timers[timerid];
             ret = get_errno(timer_getoverrun(htimer));
@@ -9677,10 +9695,10 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
     case TARGET_NR_timer_delete:
     {
         /* args: timer_t timerid */
-        target_ulong timerid = arg1;
+        target_timer_t timerid = get_timer_id(arg1);
 
-        if (timerid >= ARRAY_SIZE(g_posix_timers)) {
-            ret = -TARGET_EINVAL;
+        if (timerid < 0) {
+            ret = timerid;
         } else {
             timer_t htimer = g_posix_timers[timerid];
             ret = get_errno(timer_delete(htimer));
diff --git a/linux-user/syscall_defs.h b/linux-user/syscall_defs.h
index c9e6323..ebb3be1 100644
--- a/linux-user/syscall_defs.h
+++ b/linux-user/syscall_defs.h
@@ -2564,10 +2564,7 @@ struct target_ucred {
 
 #endif
 
-
-struct target_timer_t {
-    abi_ulong ptr;
-};
+typedef int32_t target_timer_t;
 
 #define TARGET_SIGEV_MAX_SIZE 64
 
-- 
1.7.12.4