File 0003-Drop-privileges-when-reading-the-salt-file... of Package pam_kwallet

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File 0003-Drop-privileges-when-reading-the-salt-file.patch of Package pam_kwallet (Revision 67464eed9e8efa00d5348c03ec7124ad)

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From 74403113c1cc08ef0b9c53f5ed44bdc1e57585cc Mon Sep 17 00:00:00 2001
From: Albert Astals Cid <aacid@kde.org>
Date: Sun, 27 May 2018 11:24:59 +0200
Subject: [PATCH 3/4] Drop privileges when reading the salt file

Summary:
As found by Matthias Gerstner the user here controls nearly everything:

- he controls his own password
  - he controls where the salt is read from
  - he can read the final salted hash (e.g. by calling strace() on kwalletd at
    the right time)

By using this fact he can do the following things:

- test for existence of files in locations otherwise not accessible
  - exploit an information leak. 56 bytes of root owned files will be provided
    to him in the form of a salted hash. He won't be able to easily retrieve
    the original "salt" again. But if the "salt" comes from a well structured
    input file then the possible input combinations can suddenly be quite
    limited and a brute force attack can be feasible to gain knowledge of
    certain root-owned data.
  - the fact that the user can cause a root-owned process to read 56 bytes
    from an arbitrary file in the system could have other side effects
    depending on the situation in the system. E.g. FUSE, pseudo file systems
    or device files might react specially to this.

This is a very theoretical attack, but since it's reasonable easy to fix it, let's do it :)

Test Plan: kwallet-pam still works

Reviewers: dakon

Reviewed By: dakon

Subscribers: dakon, mgerstner, fvogt, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12937
---
 pam_kwallet.c | 92 ++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 77 insertions(+), 15 deletions(-)

diff --git a/pam_kwallet.c b/pam_kwallet.c
index ecee6b3..65db960 100644
--- a/pam_kwallet.c
+++ b/pam_kwallet.c
@@ -471,7 +471,7 @@ static int better_write(int fd, const char *buffer, int len)
         writtenBytes += result;
     }
 
-    return 0;
+    return writtenBytes;
 }
 
 static void start_kwallet(pam_handle_t *pamh, struct passwd *userInfo, const char *kwalletKey)
@@ -700,6 +700,79 @@ static void createNewSalt(pam_handle_t *pamh, const char *path, struct passwd *u
     }
 }
 
+static int readSaltFile(pam_handle_t *pamh, char *path, struct passwd *userInfo, char *saltOut)
+{
+    int readSaltPipe[2];
+    if (pipe(readSaltPipe) < 0) {
+        pam_syslog(pamh, LOG_ERR, "%s: Couldn't create read salt pipes", logPrefix);
+        return 0;
+    }
+
+    const pid_t pid = fork();
+    if (pid == -1) {
+        syslog(LOG_ERR, "%s: Couldn't fork to read salt file", logPrefix);
+        close(readSaltPipe[0]);
+        close(readSaltPipe[1]);
+        return 0;
+    } else if (pid == 0) {
+        // Child process
+        close(readSaltPipe[0]); // we won't be reading from the pipe
+        if (drop_privileges(userInfo) < 0) {
+            syslog(LOG_ERR, "%s: could not set gid/uid/euid/egit for salt file reading", logPrefix);
+            free(path);
+            close(readSaltPipe[1]);
+            exit(-1);
+        }
+
+        FILE *fd = fopen(path, "r");
+        if (fd == NULL) {
+            syslog(LOG_ERR, "%s: Couldn't open file: %s because: %d-%s", logPrefix, path, errno, strerror(errno));
+            free(path);
+            close(readSaltPipe[1]);
+            exit(-1);
+        }
+        free(path);
+        char salt[KWALLET_PAM_SALTSIZE] = {};
+        const int bytesRead = fread(salt, 1, KWALLET_PAM_SALTSIZE, fd);
+        fclose(fd);
+        if (bytesRead != KWALLET_PAM_SALTSIZE) {
+            syslog(LOG_ERR, "%s: Couldn't read the full salt file contents from file. %d:%d", logPrefix, bytesRead, KWALLET_PAM_SALTSIZE);
+            exit(-1);
+        }
+
+        const ssize_t written = better_write(readSaltPipe[1], salt, KWALLET_PAM_SALTSIZE);
+
+        close(readSaltPipe[1]);
+        if (written != KWALLET_PAM_SALTSIZE) {
+            syslog(LOG_ERR, "%s: Couldn't write the full salt file contents to pipe", logPrefix);
+            exit(-1);
+        }
+
+        exit(0);
+    }
+
+    close(readSaltPipe[1]); // we won't be writting from the pipe
+
+    // pam process, just wait for child to finish
+    int status;
+    waitpid(pid, &status, 0);
+    int success = 1;
+    if (status == 0) {
+        const ssize_t readBytes = read(readSaltPipe[0], saltOut, KWALLET_PAM_SALTSIZE);
+        if (readBytes != KWALLET_PAM_SALTSIZE) {
+            pam_syslog(pamh, LOG_ERR, "%s: Couldn't read the full salt file contents from pipe", logPrefix);
+            success = 0;
+        }
+    } else {
+        pam_syslog(pamh, LOG_ERR, "%s: Couldn't read salt file", logPrefix);
+        success = 0;
+    }
+
+    close(readSaltPipe[0]);
+
+    return success;
+}
+
 int kwallet_hash(pam_handle_t *pamh, const char *passphrase, struct passwd *userInfo, char *key)
 {
     if (!gcry_check_version("1.5.0")) {
@@ -722,7 +795,6 @@ int kwallet_hash(pam_handle_t *pamh, const char *passphrase, struct passwd *user
     char *path = (char*) malloc(pathSize);
     sprintf(path, "%s/%s/%s", userInfo->pw_dir, kdehome, fixpath);
 
-    char *salt = NULL;
     if (stat(path, &info) != 0 || info.st_size == 0 || !S_ISREG(info.st_mode)) {
         createNewSalt(pamh, path, userInfo);
     }
@@ -733,19 +805,10 @@ int kwallet_hash(pam_handle_t *pamh, const char *passphrase, struct passwd *user
         return 1;
     }
 
-    FILE *fd = fopen(path, "r");
-    if (fd == NULL) {
-        syslog(LOG_ERR, "%s: Couldn't open file: %s because: %d-%s", logPrefix, path, errno, strerror(errno));
-        free(path);
-        return 1;
-    }
-    salt = (char*) malloc(KWALLET_PAM_SALTSIZE);
-    memset(salt, '\0', KWALLET_PAM_SALTSIZE);
-    fread(salt, KWALLET_PAM_SALTSIZE, 1, fd);
-    fclose(fd);
+    char salt[KWALLET_PAM_SALTSIZE] = {};
+    const int readSaltSuccess = readSaltFile(pamh, path, userInfo, salt);
     free(path);
-
-    if (salt == NULL) {
+    if (!readSaltSuccess) {
         syslog(LOG_ERR, "%s-kwalletd: Couldn't create or read the salt file", logPrefix);
         return 1;
     }
@@ -772,6 +835,5 @@ int kwallet_hash(pam_handle_t *pamh, const char *passphrase, struct passwd *user
                             salt, KWALLET_PAM_SALTSIZE,
                             KWALLET_PAM_ITERATIONS,KWALLET_PAM_KEYSIZE, key);
 
-    free(salt);
     return (int) error; // gcry_kdf_derive returns 0 on success
 }
-- 
2.17.1

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File 0003-Drop-privileges-when-reading-the-salt-file.patch of Package pam_kwallet (Revision 67464eed9e8efa00d5348c03ec7124ad)

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