CVE-2017-7501 (published 2017-11-22T22:29Z)

It was found that versions of rpm before 4.13.0.2 use temporary files with predictable names when installing an RPM. An attacker with ability to write in a directory where files will be installed could create symbolic links to an arbitrary location and modify content, and possibly permissions to arbitrary files, which could be used for denial of service or possibly privilege escalation.

Severity: MEDIUM
CVSS v2 base score: 4.6 (AV:L/AC:L/Au:N/C:P/I:P/A:P)
CVSS v3 base score: 7.8, HIGH (CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H)

Weakness information:
cwe_id: CWE-59
cwe_name: Improper Link Resolution Before File Access ('Link Following')
description: The product attempts to access a file based on the filename, but it does not properly prevent that filename from identifying a link or shortcut that resolves to an unintended resource.
extended_description: 
            
            
         
         

Fix information:
repo_url: https://github.com/rpm-software-management/rpm
commit_details: repo_url: https://github.com/rpm-software-management/rpm
author: Panu Matilainen
author_date: 2017-09-19 14:46:36+03:00
msg: Don't follow symlinks on file creation (CVE-2017-7501)

Open newly created files with O_EXCL to prevent symlink tricks.
When reopening hardlinks for writing the actual content, use append
mode instead. This is compatible with the write-only permissions but
is not destructive in case we got redirected to somebody elses file,
verify the target before actually writing anything.

As these are files with the temporary suffix, errors mean a local
user with sufficient privileges to break the installation of the package
anyway is trying to goof us on purpose, don't bother trying to mend it
(we couldn't fix the hardlink case anyhow) but just bail out.

Based on a patch by Florian Festi.
repository: repo_url: https://github.com/rpm-software-management/rpm
repo_name: rpm-software-management/rpm
description: The RPM package manager
repo_language: C
owner: rpm-software-management
file_changes: filename: fsm.c
change_type: ModificationType.MODIFY
diff: @@ -206,11 +206,22 @@ static int fsmSetFCaps(const char *path, const char *captxt)
     return rc;
 }
 
+/* Check dest is the same, empty and regular file with writeonly permissions */
+static int linkSane(FD_t wfd, const char *dest)
+{
+    struct stat sb, lsb;
+
+    return (fstat(Fileno(wfd), &sb) == 0 && sb.st_size == 0 &&
+	    (sb.st_mode & ~S_IFMT) == S_IWUSR &&
+	    lstat(dest, &lsb) == 0 && S_ISREG(lsb.st_mode) &&
+	    sb.st_dev == lsb.st_dev && sb.st_ino == lsb.st_ino);
+}
+
 /** \ingroup payload
  * Create file from payload stream.
  * @return		0 on success
  */
-static int expandRegular(rpmfi fi, const char *dest, rpmpsm psm, int nodigest, int nocontent)
+static int expandRegular(rpmfi fi, const char *dest, rpmpsm psm, int exclusive, int nodigest, int nocontent)
 {
     FD_t wfd = NULL;
     int rc = 0;
@@ -218,8 +229,14 @@ static int expandRegular(rpmfi fi, const char *dest, rpmpsm psm, int nodigest, i
     /* Create the file with 0200 permissions (write by owner). */
     {
 	mode_t old_umask = umask(0577);
-	wfd = Fopen(dest, "w.ufdio");
+	wfd = Fopen(dest, exclusive ? "wx.ufdio" : "a.ufdio");
 	umask(old_umask);
+
+	/* If reopening, make sure the file is what we expect */
+	if (!exclusive && wfd != NULL && !linkSane(wfd, dest)) {
+	    rc = RPMERR_OPEN_FAILED;
+	    goto exit;
+	}
     }
     if (Ferror(wfd)) {
 	rc = RPMERR_OPEN_FAILED;
@@ -248,7 +265,7 @@ static int fsmMkfile(rpmfi fi, const char *dest, rpmfiles files,
 	/* Create first hardlinked file empty */
 	if (*firsthardlink < 0) {
 	    *firsthardlink = rpmfiFX(fi);
-	    rc = expandRegular(fi, dest, psm, nodigest, 1);
+	    rc = expandRegular(fi, dest, psm, 1, nodigest, 1);
 	} else {
 	    /* Create hard links for others */
 	    char *fn = rpmfilesFN(files, *firsthardlink);
@@ -263,10 +280,10 @@ static int fsmMkfile(rpmfi fi, const char *dest, rpmfiles files,
        existing) file with content */
     if (numHardlinks<=1) {
 	if (!rc)
-	    rc = expandRegular(fi, dest, psm, nodigest, 0);
+	    rc = expandRegular(fi, dest, psm, 1, nodigest, 0);
     } else if (rpmfiArchiveHasContent(fi)) {
 	if (!rc)
-	    rc = expandRegular(fi, dest, psm, nodigest, 0);
+	    rc = expandRegular(fi, dest, psm, 0, nodigest, 0);
 	*firsthardlink = -1;
     } else {
 	*setmeta = 0;
@@ -939,7 +956,7 @@ int rpmPackageFilesInstall(rpmts ts, rpmte te, rpmfiles files,
 	    /* we skip the hard linked file containing the content */
 	    /* write the content to the first used instead */
 	    char *fn = rpmfilesFN(files, firsthardlink);
-	    rc = expandRegular(fi, fn, psm, nodigest, 0);
+	    rc = expandRegular(fi, fn, psm, 0, nodigest, 0);
 	    firsthardlink = -1;
 	    free(fn);
 	}

programming_language: C