dynamic-stack-buffer-overflow in apt-pkg/contrib/cmdnline.cc

Hello mainteiners!

When apt is built with clang-19 and ASAN sanitizer error
dynamic-stack-buffer-overflow arises in apt-pkg/contrib/cmdnline.cc in
CommandLine::SaveInConfig() because length becomes equal to
100 + argc * 50 on the last iteration of the for loop at line 402.

Sanitizer report:

==6090==ERROR: AddressSanitizer: dynamic-stack-buffer-overflow on 
address 0x7ffc1e99003a at pc 0x73a05287672c bp 0x7ffc1e98ff10 sp 
0x7ffc1e98ff08
WRITE of size 1 at 0x7ffc1e99003a thread T0
     #0 0x73a05287672b in CommandLine::SaveInConfig(unsigned int const&, 
char const* const*) /apt_ASAN/apt-pkg/contrib/cmndline.cc:428:23
     #1 0x73a052871043 in CommandLine::Parse(int, char const**) 
/apt_ASAN/apt-pkg/contrib/cmndline.cc:192:4
     #2 0x73a052ee359d in ParseCommandLine(CommandLine&, APT_CMD, 
Configuration* const*, pkgSystem**, int, char const**, bool 
(*)(CommandLine&), std::vector<aptDispatchWithHelp, 
std::allocator<aptDispatchWithHelp>> (*)()) 
/apt_ASAN/apt-private/private-cmndline.cc:557:13
     #3 0x5d93f23de7c8 in main /apt_ASAN/cmdline/apt.cc:106:22
     #4 0x73a051f69249 in __libc_start_call_main 
csu/../sysdeps/nptl/libc_start_call_main.h:58:16
     #5 0x73a051f69304 in __libc_start_main csu/../csu/libc-start.c:360:3
     #6 0x5d93f22f91a0 in _start (/apt_ASAN/cmdline/apt+0x6d1a0) 
(BuildId: 4b9c96fa8ff70664cdda54c4eb53fc3413f5c349)

Address 0x7ffc1e99003a is located in stack of thread T0
SUMMARY: AddressSanitizer: dynamic-stack-buffer-overflow 
/apt_ASAN/apt-pkg/contrib/cmndline.cc:428:23 in 
CommandLine::SaveInConfig(unsigned int const&, char const* const*)
Shadow bytes around the buggy address:
   0x7ffc1e98fd80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   0x7ffc1e98fe00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   0x7ffc1e98fe80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   0x7ffc1e98ff00: 00 00 00 00 ca ca ca ca 00 00 00 00 00 00 00 00
   0x7ffc1e98ff80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x7ffc1e990000: 00 00 00 00 00 00 00[02]cb cb cb cb 00 00 00 00
   0x7ffc1e990080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   0x7ffc1e990100: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   0x7ffc1e990180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   0x7ffc1e990200: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   0x7ffc1e990280: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
   Addressable:           00
   Partially addressable: 01 02 03 04 05 06 07
   Heap left redzone:       fa
   Freed heap region:       fd
   Stack left redzone:      f1
   Stack mid redzone:       f2
   Stack right redzone:     f3
   Stack after return:      f5
   Stack use after scope:   f8
   Global redzone:          f9
   Global init order:       f6
   Poisoned by user:        f7
   Container overflow:      fc
   Array cookie:            ac
   Intra object redzone:    bb
   ASan internal:           fe
   Left alloca redzone:     ca
   Right alloca redzone:    cb
==6090==ABORTING

Reproducing:

To reproduce build the project with ASAN and in cmdline/  do

./apt 0o 
"000000000000==================================================================================================================== 
"

Quotation marks are present to preserve a space in argv[2].

Suggested fixes:

I don't know how exactly to fix it, but there are a couple suggestions:
Option 1) Add an extra length check.
Diff:
--- a/apt-pkg/contrib/cmndline.cc
+++ b/apt-pkg/contrib/cmndline.cc
@@ -417,6 +417,8 @@ void CommandLine::SaveInConfig(unsigned int const 
&argc, char const * const * co
             closeQuote = true;
          }
        }
+      if (length >= sizeof(cmdline) -2)
+         continue;
        if (closeQuote == true)
        {
          cmdline[length++] = '\'';

Option 2) Allocate more memory for the cmdline variable based on
           argv length (could be excessive in the suggestion below).
Diff:

--- a/apt-pkg/contrib/cmndline.cc
+++ b/apt-pkg/contrib/cmndline.cc
@@ -394,7 +394,11 @@ bool CommandLine::DispatchArg(Dispatch const * 
const Map,bool NoMatch)
     than nothing after all. */
  void CommandLine::SaveInConfig(unsigned int const &argc, char const * 
const * const argv)
  {
-   char cmdline[100 + argc * 50];
+   unsigned int argvLength = 0;
+   for (int i = 0; i < argc; i++)
+   {
+     argvLength += strlen(argv[i]) + 1;
+   }
+   char cmdline[100 + argvLength * 3];
     memset(cmdline, 0, sizeof(cmdline));
     unsigned int length = 0;
     bool lastWasOption = false;