[PATCH] linux-user: Prevent crash in epoll_ctl

[LemonBoy]

>From 894bb5172705e46a3a04c93b4962c0f0cafee814 Mon Sep 17 00:00:00 2001
From: Giuseppe Musacchio <address@hidden>
Date: Fri, 17 Apr 2020 17:25:07 +0200
Subject: [PATCH] linux-user: Prevent crash in epoll_ctl

The `event` parameter is ignored by the kernel if `op` is EPOLL_CTL_DEL,
do the same and avoid returning EFAULT if garbage is passed instead of a
valid pointer.

Signed-off-by: Giuseppe Musacchio <address@hidden>
---
 linux-user/syscall.c | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 674f70e70a..a51ff43f9b 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -12020,17 +12020,25 @@ static abi_long do_syscall1(void *cpu_env, int num, 
abi_long arg1,
         struct epoll_event ep;
         struct epoll_event *epp = 0;
         if (arg4) {
-            struct target_epoll_event *target_ep;
-            if (!lock_user_struct(VERIFY_READ, target_ep, arg4, 1)) {
-                return -TARGET_EFAULT;
+            if (arg2 != EPOLL_CTL_DEL) {
+                struct target_epoll_event *target_ep;
+                if (!lock_user_struct(VERIFY_READ, target_ep, arg4, 1)) {
+                    return -TARGET_EFAULT;
+                }
+                ep.events = tswap32(target_ep->events);
+                /*
+                 * The epoll_data_t union is just opaque data to the kernel,
+                 * so we transfer all 64 bits across and need not worry what
+                 * actual data type it is.
+                 */
+                ep.data.u64 = tswap64(target_ep->data.u64);
+                unlock_user_struct(target_ep, arg4, 0);
             }
-            ep.events = tswap32(target_ep->events);
-            /* The epoll_data_t union is just opaque data to the kernel,
-             * so we transfer all 64 bits across and need not worry what
-             * actual data type it is.
+            /*
+             * before kernel 2.6.9, EPOLL_CTL_DEL operation required a
+             * non-null pointer, even though this argument is ignored.
+             *
              */
-            ep.data.u64 = tswap64(target_ep->data.u64);
-            unlock_user_struct(target_ep, arg4, 0);
             epp = &ep;
         }
         return get_errno(epoll_ctl(arg1, arg2, arg3, epp));
-- 
2.20.1