if file is inaccessible for _apt, disable privilege drop in acquire

We had a very similar method previously for our own private usage, but
with some generalisation we can move this check into the acquire system
proper so that all frontends profit from this compatibility change.

As we are disabling a security feature here a warning is issued and
frontends are advised to consider reworking their download logic if
possible.

Note that this is implemented as an all or nothing situation: We can't
just (not) drop privileges for a subset of the files in a fetcher, so in
case you have to download some files with and some without you need to
use two fetchers.

8 files changed, 60 insertions, 66 deletions

diff --git a/apt-pkg/acquire.cc b/apt-pkg/acquire.cc
index f8b077367..cb32e8f2b 100644
--- a/apt-pkg/acquire.cc
+++ b/apt-pkg/acquire.cc
@@ -34,6 +34,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
+#include <fcntl.h>
 #include <pwd.h>
 #include <grp.h>
 #include <dirent.h>
@@ -446,8 +447,58 @@ void pkgAcquire::RunFds(fd_set *RSet,fd_set *WSet)
 /* This runs the queues. It manages a select loop for all of the
    Worker tasks. The workers interact with the queues and items to
    manage the actual fetch. */
+static void CheckDropPrivsMustBeDisabled(pkgAcquire const &Fetcher)
+{
+   if(getuid() != 0)
+      return;
+
+   std::string SandboxUser = _config->Find("APT::Sandbox::User");
+   if (SandboxUser.empty())
+      return;
+
+   struct passwd const * const pw = getpwnam(SandboxUser.c_str());
+   if (pw == NULL)
+      return;
+
+   if (setegid(pw->pw_gid) != 0)
+      _error->Errno("setegid", "setegid %u failed", pw->pw_gid);
+   if (seteuid(pw->pw_uid) != 0)
+      _error->Errno("seteuid", "seteuid %u failed", pw->pw_uid);
+
+   bool dropPrivs = true;
+   for (pkgAcquire::ItemCIterator I = Fetcher.ItemsBegin();
+	I != Fetcher.ItemsEnd() && dropPrivs == true; ++I)
+   {
+      if ((*I)->DestFile.empty())
+	 continue;
+
+      // we check directory instead of file as the file might or might not
+      // exist already as a link or not which complicates everything…
+      std::string dirname = flNotFile((*I)->DestFile);
+      if (DirectoryExists(dirname))
+	 ;
+      else
+	 continue; // assume it is created correctly by the acquire system
+
+      if (faccessat(AT_FDCWD, dirname.c_str(), R_OK | W_OK | X_OK, AT_EACCESS | AT_SYMLINK_NOFOLLOW) != 0)
+      {
+	 dropPrivs = false;
+	 _error->WarningE("pkgAcquire::Run", _("Can't drop privileges for downloading as file '%s' couldn't be accessed by user '%s'."),
+	       (*I)->DestFile.c_str(), SandboxUser.c_str());
+	 _config->Set("APT::Sandbox::User", "");
+	 break;
+      }
+   }
+
+   if (seteuid(0) != 0)
+      _error->Errno("seteuid", "seteuid %u failed", 0);
+   if (setegid(0) != 0)
+      _error->Errno("setegid", "setegid %u failed", 0);
+}
 pkgAcquire::RunResult pkgAcquire::Run(int PulseIntervall)
 {
+   CheckDropPrivsMustBeDisabled(*this);
+
    Running = true;
    
    for (Queue *I = Queues; I != 0; I = I->Next)
diff --git a/apt-pkg/acquire.h b/apt-pkg/acquire.h
index 10025a6ef..3e5ca41cd 100644
--- a/apt-pkg/acquire.h
+++ b/apt-pkg/acquire.h
@@ -303,9 +303,11 @@ class pkgAcquire
 
    /** \brief Get the head of the list of items. */
    inline ItemIterator ItemsBegin() {return Items.begin();};
+   inline ItemCIterator ItemsBegin() const {return Items.begin();};
 
    /** \brief Get the end iterator of the list of items. */
    inline ItemIterator ItemsEnd() {return Items.end();};
+   inline ItemCIterator ItemsEnd() const {return Items.end();};
    
    // Iterate over queued Item URIs
    class UriIterator;
diff --git a/apt-private/private-download.cc b/apt-private/private-download.cc
index 18a9b1fbc..8a57ccc86 100644
--- a/apt-private/private-download.cc
+++ b/apt-private/private-download.cc
@@ -26,55 +26,6 @@
 #include <apti18n.h>
 									/*}}}*/
 
-bool CheckDropPrivsMustBeDisabled(pkgAcquire &Fetcher)			/*{{{*/
-{
-   // no need/possibility to drop privs
-   if(getuid() != 0)
-      return true;
-
-   // the user does not want to drop privs
-   std::string SandboxUser = _config->Find("APT::Sandbox::User");
-   if (SandboxUser.empty())
-      return true;
-
-   struct passwd const * const pw = getpwnam(SandboxUser.c_str());
-   if (pw == NULL)
-      return true;
-
-   if (seteuid(pw->pw_uid) != 0)
-      return _error->Errno("seteuid", "seteuid %u failed", pw->pw_uid);
-
-   bool res = true;
-   // check if we can write to destfile
-   for (pkgAcquire::ItemIterator I = Fetcher.ItemsBegin();
-	I != Fetcher.ItemsEnd() && res == true; ++I)
-   {
-      if ((*I)->DestFile.empty())
-	 continue;
-      // we assume that an existing (partial) file means that we have sufficient rights
-      if (RealFileExists((*I)->DestFile))
-	 continue;
-      int fd = open((*I)->DestFile.c_str(), O_CREAT | O_EXCL | O_RDWR, 0600);
-      if (fd < 0)
-      {
-	 res = false;
-	 std::string msg;
-	 strprintf(msg, _("Can't drop privileges for downloading as file '%s' couldn't be accessed by user '%s'."),
-	       (*I)->DestFile.c_str(), SandboxUser.c_str());
-	 std::cerr << "W: " << msg << std::endl;
-	 _config->Set("APT::Sandbox::User", "");
-	 break;
-      }
-      unlink((*I)->DestFile.c_str());
-      close(fd);
-   }
-
-   if (seteuid(0) != 0)
-      return _error->Errno("seteuid", "seteuid %u failed", 0);
-
-   return res;
-}
-									/*}}}*/
 // CheckAuth - check if each download comes form a trusted source	/*{{{*/
 bool CheckAuth(pkgAcquire& Fetcher, bool const PromptUser)
 {
diff --git a/apt-private/private-download.h b/apt-private/private-download.h
index 0f3db5e7a..80643e0f2 100644
--- a/apt-private/private-download.h
+++ b/apt-private/private-download.h
@@ -8,8 +8,6 @@
 
 class pkgAcquire;
 
-APT_PUBLIC bool CheckDropPrivsMustBeDisabled(pkgAcquire &Fetcher);
-
 // Check if all files in the fetcher are authenticated
 APT_PUBLIC bool CheckAuth(pkgAcquire& Fetcher, bool const PromptUser);
 
diff --git a/cmdline/apt-get.cc b/cmdline/apt-get.cc
index d3b3da240..ebc8c94c2 100644
--- a/cmdline/apt-get.cc
+++ b/cmdline/apt-get.cc
@@ -629,9 +629,6 @@ static bool DoDownload(CommandLine &CmdL)
       return true;
    }
 
-   // Disable drop-privs if "_apt" can not write to the target dir
-   CheckDropPrivsMustBeDisabled(Fetcher);
-
    if (_error->PendingError() == true || CheckAuth(Fetcher, false) == false)
       return false;
 
@@ -850,9 +847,6 @@ static bool DoSource(CommandLine &CmdL)
       return true;
    }
 
-   // Disable drop-privs if "_apt" can not write to the target dir
-   CheckDropPrivsMustBeDisabled(Fetcher);
-
    // check authentication status of the source as well
    if (UntrustedList.empty() == false && AuthPrompt(UntrustedList, false) == false)
       return false;
@@ -1403,8 +1397,6 @@ static bool DoChangelog(CommandLine &CmdL)
 
    if (printOnly == false)
    {
-      // Note: CheckDropPrivsMustBeDisabled isn't needed here as the download happens in a dedicated tempdir
-
       bool Failed = false;
       if (AcquireRun(Fetcher, 0, &Failed, NULL) == false || Failed == true)
 	 return false;
diff --git a/cmdline/apt-helper.cc b/cmdline/apt-helper.cc
index d235ac315..3c49bf149 100644
--- a/cmdline/apt-helper.cc
+++ b/cmdline/apt-helper.cc
@@ -68,9 +68,6 @@ static bool DoDownloadFile(CommandLine &CmdL)
       fileind += 3;
    }
 
-   // Disable drop-privs if "_apt" can not write to the target dir
-   CheckDropPrivsMustBeDisabled(Fetcher);
-
    bool Failed = false;
    if (AcquireRun(Fetcher, 0, &Failed, NULL) == false || Failed == true)
       return _error->Error(_("Download Failed"));
diff --git a/test/integration/test-acquire-same-file-multiple-times b/test/integration/test-acquire-same-file-multiple-times
index 526765521..a850f897c 100755
--- a/test/integration/test-acquire-same-file-multiple-times
+++ b/test/integration/test-acquire-same-file-multiple-times
@@ -14,10 +14,9 @@ filedown() {
 	msgtest 'Downloading the same URI twice over file' "$1"
 	testsuccess --nomsg apthelper download-file file:///$APTARCHIVE/foo ./downloaded/foo1 '' file:///$APTARCHIVE/foo ./downloaded/foo2 '' -o Debug::pkgAcquire::Worker=1
 	cp rootdir/tmp/testsuccess.output download.log
-	#cat download.log
 	testsuccess cmp $TESTFILE ./downloaded/foo1
 	testsuccess cmp ./downloaded/foo1 ./downloaded/foo2
-	#testequal '1' grep -c '200%20URI%20Start' ./download.log
+	testequal '1' grep -c '200%20URI%20Start' ./download.log
 	testequal '1' grep -c '201%20URI%20Done' ./download.log
 	rm -f ./downloaded/foo1 ./downloaded/foo2
 }
diff --git a/test/integration/test-apt-get-changelog b/test/integration/test-apt-get-changelog
index 6ca05d0fa..502920617 100755
--- a/test/integration/test-apt-get-changelog
+++ b/test/integration/test-apt-get-changelog
@@ -47,8 +47,12 @@ testsuccessequal "'http://localhost:8080/main/f/foo/foo_1.0.changelog' foo.chang
 'http://localhost:8080/main/libb/libbar/libbar_1.0.changelog' libbar.changelog" aptget changelog foo libbar --print-uris -o Acquire::Changelogs::URI::Override::Label::Debian='http://localhost:8080/CHANGEPATH.changelog'
 
 releasechanger 'Changelogs' 'no'
-testequal 'E: Failed to fetch changelog:/foo.changelog  Changelog unavailable for foo=1.0
-' aptget changelog foo -qq -d
+if [ "$(id -u)" = '0' ]; then
+	testfailuremsg "W: Can't drop privileges for downloading as file 'foo.changelog' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
+E: Failed to fetch changelog:/foo.changelog  Changelog unavailable for foo=1.0" aptget changelog foo -d
+else
+	testfailuremsg 'E: Failed to fetch changelog:/foo.changelog  Changelog unavailable for foo=1.0' aptget changelog foo -d
+fi
 
 sed -i '/^Changelogs: / d' $(find rootdir/var/lib/apt/lists -name '*Release')
 releasechanger 'Label' 'Testcases'