From 15240b29d55cfd775221cc6482407c1172e2a5a1 Mon Sep 17 00:00:00 2001
From: Lukas Slebodnik <lslebodn@redhat.com>
Date: Fri, 21 Nov 2014 11:28:36 +0100
Subject: [PATCH 106/112] sss_client: Fix race condition in memory cache
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Thread safe initialisation was fixed in ticket #2380, but there is
still race condition in reinitialisation.

If caches is invalidated with command sss_cache -U (-G or -E) then
client code will need to reinitialize fast memory cache.
Let say we have two threads. The 1st thread find out that memory cache
should be reinitialized; therefore the fast memory cached is unmapped
and context destroyed. In the same time, 2nd thread tried to check
header of memory cache whether it is initialized and valid. As a result
of previously unmapped memory the 2nd thread access
out of bound memory (SEGFAULT).

The destroying of fast memory cache cannot be done any time. We need
to be sure that there isn't any other thread which uses mmaped memory.
The new counter of active threads was added for this purpose. The state
of fast memory cache was converted from boolean to three value state
(UNINITIALIZED, INITIALIZED, RECYCLED)
UNINITIALIZED
    - the fast memory cache need to be initialized.
    - if there is a problem with initialisation the state will not change
    - after successful initialisation, the state will change to INITIALIZED
INITIALIZED
    - if the cahe was invalidated or there is any other problem was
      detected in memory cache header the state will change to RECYCLED
      and memory cache IS NOT destroyed.
RECYCLED
    - nothing will be done is there are any active threads which may use
      the data from mmaped memory
    - if there aren't active threads the fast memory cahe is destroyed and
      state is changed to UNINITIALIZED.

https://fedorahosted.org/sssd/ticket/2445

Reviewed-by: Michal Židek <mzidek@redhat.com>
---
 src/sss_client/nss_mc.h        | 10 ++++++++-
 src/sss_client/nss_mc_common.c | 46 ++++++++++++++++++++++++++++++++++--------
 src/sss_client/nss_mc_group.c  |  8 ++++++--
 src/sss_client/nss_mc_passwd.c |  8 ++++++--
 4 files changed, 59 insertions(+), 13 deletions(-)

diff --git a/src/sss_client/nss_mc.h b/src/sss_client/nss_mc.h
index 685cc41c0530750d890050f0917dc88be14d96ea..050bd4100dec091cb096a7d97bfe6615b12654da 100644
--- a/src/sss_client/nss_mc.h
+++ b/src/sss_client/nss_mc.h
@@ -33,9 +33,15 @@
 typedef int errno_t;
 #endif
 
+enum sss_mc_state {
+    UNINITIALIZED = 0,
+    INITIALIZED,
+    RECYCLED,
+};
+
 /* common stuff */
 struct sss_cli_mc_ctx {
-    bool initialized;
+    enum sss_mc_state initialized;
     int fd;
 
     uint32_t seed;          /* seed from the tables header */
@@ -48,6 +54,8 @@ struct sss_cli_mc_ctx {
 
     uint32_t *hash_table;   /* hash table address (in mmap) */
     uint32_t ht_size;       /* size of hash table */
+
+    uint32_t active_threads; /* count of threads which use memory cache */
 };
 
 errno_t sss_nss_mc_get_ctx(const char *name, struct sss_cli_mc_ctx *ctx);
diff --git a/src/sss_client/nss_mc_common.c b/src/sss_client/nss_mc_common.c
index 9c6e1af1642275fc7738b51d7ca80d712d49b2ac..89ff6b46e2abee03039cfd632ef50231eab92eec 100644
--- a/src/sss_client/nss_mc_common.c
+++ b/src/sss_client/nss_mc_common.c
@@ -123,7 +123,7 @@ static errno_t sss_nss_mc_init_ctx(const char *name,
 
     sss_nss_lock();
     /* check if ctx is initialised by previous thread. */
-    if (ctx->initialized) {
+    if (ctx->initialized != UNINITIALIZED) {
         ret = sss_nss_check_header(ctx);
         goto done;
     }
@@ -163,7 +163,7 @@ static errno_t sss_nss_mc_init_ctx(const char *name,
         goto done;
     }
 
-    ctx->initialized = true;
+    ctx->initialized = INITIALIZED;
 
     ret = 0;
 
@@ -181,22 +181,52 @@ errno_t sss_nss_mc_get_ctx(const char *name, struct sss_cli_mc_ctx *ctx)
 {
     char *envval;
     int ret;
+    bool need_decrement = false;
 
     envval = getenv("SSS_NSS_USE_MEMCACHE");
     if (envval && strcasecmp(envval, "NO") == 0) {
         return EPERM;
     }
 
-    if (ctx->initialized) {
+    switch (ctx->initialized) {
+    case UNINITIALIZED:
+        __sync_add_and_fetch(&ctx->active_threads, 1);
+        ret = sss_nss_mc_init_ctx(name, ctx);
+        if (ret) {
+            need_decrement = true;
+        }
+        break;
+    case INITIALIZED:
+        __sync_add_and_fetch(&ctx->active_threads, 1);
         ret = sss_nss_check_header(ctx);
-        goto done;
+        if (ret) {
+            need_decrement = true;
+        }
+        break;
+    case RECYCLED:
+        /* we need to safely destroy memory cache */
+        ret = EAGAIN;
+        break;
+    default:
+        ret = EFAULT;
     }
 
-    ret = sss_nss_mc_init_ctx(name, ctx);
-
-done:
     if (ret) {
-        sss_nss_mc_destroy_ctx(ctx);
+        if (ctx->initialized == INITIALIZED) {
+            ctx->initialized = RECYCLED;
+        }
+        if (ctx->initialized == RECYCLED && ctx->active_threads == 0) {
+            /* just one thread should call munmap */
+            sss_nss_lock();
+            if (ctx->initialized == RECYCLED) {
+                sss_nss_mc_destroy_ctx(ctx);
+            }
+            sss_nss_unlock();
+        }
+        if (need_decrement) {
+            /* In case of error, we will not touch mmapped area => decrement */
+            __sync_sub_and_fetch(&ctx->active_threads, 1);
+        }
     }
     return ret;
 }
diff --git a/src/sss_client/nss_mc_group.c b/src/sss_client/nss_mc_group.c
index 268b40ef02f2a621c4f61755ce4dfe2c3786bfa6..e0fdb97f628ac19741409be29566e4af5a391f74 100644
--- a/src/sss_client/nss_mc_group.c
+++ b/src/sss_client/nss_mc_group.c
@@ -29,7 +29,8 @@
 #include "nss_mc.h"
 #include "util/util_safealign.h"
 
-struct sss_cli_mc_ctx gr_mc_ctx = { false, -1, 0, NULL, 0, NULL, 0, NULL, 0 };
+struct sss_cli_mc_ctx gr_mc_ctx = { UNINITIALIZED, -1, 0, NULL, 0, NULL, 0,
+                                    NULL, 0, 0 };
 
 static errno_t sss_nss_mc_parse_result(struct sss_mc_rec *rec,
                                        struct group *result,
@@ -176,6 +177,7 @@ errno_t sss_nss_mc_getgrnam(const char *name, size_t name_len,
 
 done:
     free(rec);
+    __sync_sub_and_fetch(&gr_mc_ctx.active_threads, 1);
     return ret;
 }
 
@@ -198,7 +200,8 @@ errno_t sss_nss_mc_getgrgid(gid_t gid,
 
     len = snprintf(gidstr, 11, "%ld", (long)gid);
     if (len > 10) {
-        return EINVAL;
+        ret = EINVAL;
+        goto done;
     }
 
     /* hashes are calculated including the NULL terminator */
@@ -242,6 +245,7 @@ errno_t sss_nss_mc_getgrgid(gid_t gid,
 
 done:
     free(rec);
+    __sync_sub_and_fetch(&gr_mc_ctx.active_threads, 1);
     return ret;
 }
 
diff --git a/src/sss_client/nss_mc_passwd.c b/src/sss_client/nss_mc_passwd.c
index fa19afc3c0e468430183ed3f13b80e086251ee01..10e43e2af43c5e7f1738e281b3ed260d89f3a004 100644
--- a/src/sss_client/nss_mc_passwd.c
+++ b/src/sss_client/nss_mc_passwd.c
@@ -28,7 +28,8 @@
 #include <time.h>
 #include "nss_mc.h"
 
-struct sss_cli_mc_ctx pw_mc_ctx = { false, -1, 0, NULL, 0, NULL, 0, NULL, 0 };
+struct sss_cli_mc_ctx pw_mc_ctx = { UNINITIALIZED, -1, 0, NULL, 0, NULL, 0,
+                                    NULL, 0, 0 };
 
 static errno_t sss_nss_mc_parse_result(struct sss_mc_rec *rec,
                                        struct passwd *result,
@@ -170,6 +171,7 @@ errno_t sss_nss_mc_getpwnam(const char *name, size_t name_len,
 
 done:
     free(rec);
+    __sync_sub_and_fetch(&pw_mc_ctx.active_threads, 1);
     return ret;
 }
 
@@ -192,7 +194,8 @@ errno_t sss_nss_mc_getpwuid(uid_t uid,
 
     len = snprintf(uidstr, 11, "%ld", (long)uid);
     if (len > 10) {
-        return EINVAL;
+        ret = EINVAL;
+        goto done;
     }
 
     /* hashes are calculated including the NULL terminator */
@@ -236,6 +239,7 @@ errno_t sss_nss_mc_getpwuid(uid_t uid,
 
 done:
     free(rec);
+    __sync_sub_and_fetch(&pw_mc_ctx.active_threads, 1);
     return ret;
 }
 
-- 
1.9.3