do not use _apt for file/copy sources if it isn't world-accessible

In 0940230d we started dropping privileges for file (and a bit later for
copy, too) with the intend of uniforming this for all methods. The
commit message says that the source will likely fail based on the
compressors already – and there isn't much secret in the repository
content. After all, after apt has run the update everyone can access the
content via apt anyway…

There are sources through which worked before which are mostly
single-deb (and those with the uncompressed files available).
The first one being especially surprising for users maybe, so instead of
failing, we make it so that apt detects that it can't access a source as
_apt and if so doesn't drop (for all sources!) privileges – but we limit
this to file/copy, so the uncompress which might be needed will still
fail – but that failed before this regression.

We display a notice about this, mostly so that if it still fails (e.g.
compressed) the user has some idea what is wrong.

Closes: 805069

1 files changed, 84 insertions, 29 deletions

diff --git a/apt-pkg/acquire.cc b/apt-pkg/acquire.cc
index 81faee5d8..fb1210750 100644
--- a/apt-pkg/acquire.cc
+++ b/apt-pkg/acquire.cc
@@ -30,6 +30,7 @@
 #include <iostream>
 #include <sstream>
 #include <iomanip>
+#include <memory>
 
 #include <stdio.h>
 #include <stdlib.h>
@@ -76,7 +77,7 @@ void pkgAcquire::Initialize()
 
    // chown the auth.conf file as it will be accessed by our methods
    std::string const SandboxUser = _config->Find("APT::Sandbox::User");
-   if (getuid() == 0 && SandboxUser.empty() == false) // if we aren't root, we can't chown, so don't try it
+   if (getuid() == 0 && SandboxUser.empty() == false && SandboxUser != "root") // if we aren't root, we can't chown, so don't try it
    {
       struct passwd const * const pw = getpwnam(SandboxUser.c_str());
       struct group const * const gr = getgrnam("root");
@@ -102,7 +103,7 @@ static bool SetupAPTPartialDirectory(std::string const &grand, std::string const
       return false;
 
    std::string const SandboxUser = _config->Find("APT::Sandbox::User");
-   if (getuid() == 0 && SandboxUser.empty() == false) // if we aren't root, we can't chown, so don't try it
+   if (getuid() == 0 && SandboxUser.empty() == false && SandboxUser != "root") // if we aren't root, we can't chown, so don't try it
    {
       struct passwd const * const pw = getpwnam(SandboxUser.c_str());
       struct group const * const gr = getgrnam("root");
@@ -448,13 +449,50 @@ 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 bool IsAccessibleBySandboxUser(std::string const &filename, bool const ReadWrite)
+{
+   // you would think this is easily to answer with faccessat, right? Wrong!
+   // It e.g. gets groups wrong, so the only thing which works reliable is trying
+   // to open the file we want to open later on…
+   if (unlikely(filename.empty()))
+      return true;
+
+   if (ReadWrite == false)
+   {
+      errno = 0;
+      // can we read a file? Note that non-existing files are "fine"
+      int const fd = open(filename.c_str(), O_RDONLY | O_CLOEXEC);
+      if (fd == -1 && errno == EACCES)
+	 return false;
+      close(fd);
+      return true;
+   }
+   else
+   {
+      // the file might not exist yet and even if it does we will fix permissions,
+      // so important is here just that the directory it is in allows that
+      std::string const dirname = flNotFile(filename);
+      if (unlikely(dirname.empty()))
+	 return true;
+
+      char const * const filetag = ".apt-acquire-privs-test.XXXXXX";
+      std::string const tmpfile_tpl = flCombine(dirname, filetag);
+      std::unique_ptr<char, decltype(std::free) *> tmpfile { strdup(tmpfile_tpl.c_str()), std::free };
+      int const fd = mkstemp(tmpfile.get());
+      if (fd == -1 && errno == EACCES)
+	 return false;
+      RemoveFile("IsAccessibleBySandboxUser", tmpfile.get());
+      close(fd);
+      return true;
+   }
+}
 static void CheckDropPrivsMustBeDisabled(pkgAcquire const &Fetcher)
 {
    if(getuid() != 0)
       return;
 
-   std::string SandboxUser = _config->Find("APT::Sandbox::User");
-   if (SandboxUser.empty())
+   std::string const SandboxUser = _config->Find("APT::Sandbox::User");
+   if (SandboxUser.empty() || SandboxUser == "root")
       return;
 
    struct passwd const * const pw = getpwnam(SandboxUser.c_str());
@@ -463,50 +501,67 @@ static void CheckDropPrivsMustBeDisabled(pkgAcquire const &Fetcher)
 
    gid_t const old_euid = geteuid();
    gid_t const old_egid = getegid();
+
+   long const ngroups_max = sysconf(_SC_NGROUPS_MAX);
+   std::unique_ptr<gid_t[]> old_gidlist(new gid_t[ngroups_max]);
+   if (unlikely(old_gidlist == NULL))
+      return;
+   ssize_t old_gidlist_nr;
+   if ((old_gidlist_nr = getgroups(ngroups_max, old_gidlist.get())) < 0)
+   {
+      _error->FatalE("getgroups", "getgroups %lu failed", ngroups_max);
+      old_gidlist[0] = 0;
+      old_gidlist_nr = 1;
+   }
+   if (setgroups(1, &pw->pw_gid))
+      _error->FatalE("setgroups", "setgroups %u failed", pw->pw_gid);
+
    if (setegid(pw->pw_gid) != 0)
-      _error->Errno("setegid", "setegid %u failed", pw->pw_gid);
+      _error->FatalE("setegid", "setegid %u failed", pw->pw_gid);
    if (seteuid(pw->pw_uid) != 0)
-      _error->Errno("seteuid", "seteuid %u failed", pw->pw_uid);
+      _error->FatalE("seteuid", "seteuid %u failed", pw->pw_uid);
 
    for (pkgAcquire::ItemCIterator I = Fetcher.ItemsBegin();
 	I != Fetcher.ItemsEnd(); ++I)
    {
-      std::string filename = (*I)->DestFile;
-      if (filename.empty())
-	 continue;
-
       // no need to drop privileges for a complete file
       if ((*I)->Complete == true)
 	 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(filename);
-      if (unlikely(dirname.empty()))
-	 continue;
-      // translate relative to absolute for DirectoryExists
-      // FIXME: What about ../ and ./../ ?
-      if (dirname.substr(0,2) == "./")
-	 dirname = SafeGetCWD() + dirname.substr(2);
-
-      if (DirectoryExists(dirname))
-	 ;
-      else
-	 continue; // assume it is created correctly by the acquire system
-
-      if (faccessat(-1, dirname.c_str(), R_OK | W_OK | X_OK, AT_EACCESS | AT_SYMLINK_NOFOLLOW) != 0)
+      // if destination file is inaccessible all hope is lost for privilege dropping
+      if (IsAccessibleBySandboxUser((*I)->DestFile, true) == false)
       {
 	 _error->WarningE("pkgAcquire::Run", _("Can't drop privileges for downloading as file '%s' couldn't be accessed by user '%s'."),
-	       filename.c_str(), SandboxUser.c_str());
+	       (*I)->DestFile.c_str(), SandboxUser.c_str());
 	 _config->Set("APT::Sandbox::User", "");
 	 break;
       }
+
+      // if its the source file (e.g. local sources) we might be lucky
+      // by dropping the dropping only for some methods.
+      URI const source = (*I)->DescURI();
+      if (source.Access == "file" || source.Access == "copy")
+      {
+	 std::string const conf = "Binary::" + source.Access + "::APT::Sandbox::User";
+	 if (_config->Exists(conf) == true)
+	    continue;
+
+	 if (IsAccessibleBySandboxUser(source.Path, false) == false)
+	 {
+	    _error->NoticeE("pkgAcquire::Run", _("Can't drop privileges for downloading as file '%s' couldn't be accessed by user '%s'."),
+		  source.Path.c_str(), SandboxUser.c_str());
+	    _config->CndSet("Binary::file::APT::Sandbox::User", "root");
+	    _config->CndSet("Binary::copy::APT::Sandbox::User", "root");
+	 }
+      }
    }
 
    if (seteuid(old_euid) != 0)
-      _error->Errno("seteuid", "seteuid %u failed", old_euid);
+      _error->FatalE("seteuid", "seteuid %u failed", old_euid);
    if (setegid(old_egid) != 0)
-      _error->Errno("setegid", "setegid %u failed", old_egid);
+      _error->FatalE("setegid", "setegid %u failed", old_egid);
+   if (setgroups(old_gidlist_nr, old_gidlist.get()))
+      _error->FatalE("setgroups", "setgroups %u failed", 0);
 }
 pkgAcquire::RunResult pkgAcquire::Run(int PulseIntervall)
 {