CVE-2024-53680: Linux Kernel Patch Resolves IPVS Vulnerability and Uninitialized Stack Access

ipvs: fix UB due to uninitialized stack access in ip_vs_protocol_init()

Under certain kernel configurations when building with Clang/LLVM, the compiler does not generate a return or jump as the terminator instruction for ip_vs_protocol_init(), triggering the following objtool warning during build time:

  vmlinux.o: warning: objtool: ip_vs_protocol_init() falls through to next function __initstub__kmod_ip_vs_rr__935_123_ip_vs_rr_init6()

At runtime, this either causes an oops when trying to load the ipvs module or a boot-time panic if ipvs is built-in. This same issue has been reported by the Intel kernel test robot previously(see link here).

Digging deeper into both LLVM and the kernel code reveals this to be an undefined behavior problem. ip_vs_protocol_init() uses an on-stack buffer of 64 chars to store the registered protocol names and leaves it uninitialized after definition. The function calls strnlen() when concatenating protocol names into the buffer. With CONFIG_FORTIFY_SOURCE, strnlen() performs an extra step to check whether the last byte of the input char buffer is a null character (commit 3009f891bb9f ["fortify: Allow strlen() and strnlen() to pass compile-time known lengths"]).

This, together with possibly other configurations, causes the following IR to be generated

  define hidden i32 @ip_vs_protocol_init() local_unnamed_addr #5 section ".init.text" align 16 !kcfi_type !29 {
    %1 = alloca [64 x i8], align 16
    ...
  14:                                               ; preds = %11
    %15 = getelementptr inbounds i8, ptr %1, i64 63
    %16 = load i8, ptr %15, align 1
    %17 = tail call i1 @llvm.is.constant.i8(i8 %16)
    %18 = icmp eq i8 %16, 
    %19 = select i1 %17, i1 %18, i1 false
    br i1 %19, label %20, label %23
  20:                                               ; preds = %14
    %21 = call i64 @strlen(ptr noundef nonnull dereferenceable(1) %1) #23
    ...
  23:                                               ; preds = %14, %11, %20
    %24 = call i64 @strnlen(ptr noundef nonnull dereferenceable(1) %1, i64 noundef 64) #24
    ...
  }

The above code calculates the address of the last char in the buffer (value %15) and then loads from it (value %16). Because the buffer is never initialized, the LLVM GVN pass marks value %16 as undefined:

  %13 = getelementptr inbounds i8, ptr %1, i64 63
  br i1 undef, label %14, label %17

This gives later passes (SCCP, in particular) more DCE opportunities by propagating the undef value further, and eventually removes everything after the load on the uninitialized stack location:

  define hidden i32 @ip_vs_protocol_init() local_unnamed_addr # section ".init.text" align 16 !kcfi_type !11 {
    %1 = alloca [64 x i8], align 16
    ...
  12:                                               ; preds = %11
    %13 = getelementptr inbounds i8, ptr %1, i64 63
    unreachable
  }

In this way, the generated native code will just fall through to the next function, as LLVM does not generate any code for the unreachable IR instruction and leaves the function without a terminator.

To fix this issue, zero the on-stack buffer to avoid this possible undefined behavior by initializing it with null characters. This simple fix ensures that the proper terminator instruction is generated for the ip_vs_protocol_init() function, which helps prevent further problems when trying to load the ipvs module or during boot-up when ipvs is built-in.

This patch resolves the vulnerability associated with CVE-2024-53680 and ensures the proper termination for functions related to the ipvs module.

Timeline

Published on: 01/11/2025 13:15:25 UTC
Last modified on: 01/20/2025 06:21:55 UTC