Validate that ELF objects request a non-executable stack (#1070)

Closes #1069.

Author: geofft

src/validation.rs

@@ -10,12 +10,12 @@ use {
     object::{
         Architecture, Endianness, FileKind, Object, SectionIndex, SymbolScope,
         elf::{
-            ET_DYN, ET_EXEC, FileHeader32, FileHeader64, SHN_UNDEF, STB_GLOBAL, STB_WEAK,
-            STV_DEFAULT, STV_HIDDEN,
+            ET_DYN, ET_EXEC, FileHeader32, FileHeader64, PF_X, PT_GNU_STACK, SHN_UNDEF, STB_GLOBAL,
+            STB_WEAK, STV_DEFAULT, STV_HIDDEN,
         },
         macho::{LC_CODE_SIGNATURE, MH_OBJECT, MH_TWOLEVEL, MachHeader32, MachHeader64},
         read::{
-            elf::{Dyn, FileHeader, SectionHeader, Sym},
+            elf::{Dyn, FileHeader, ProgramHeader, SectionHeader, Sym},
             macho::{LoadCommandVariant, MachHeader, Nlist, Section, Segment},
             pe::{ImageNtHeaders, PeFile, PeFile32, PeFile64},
         },
@@ -1161,6 +1161,39 @@ fn validate_elf<Elf: FileHeader<Endian = Endianness>>(
         }
     }
 
+    // Verify that objects are not requesting an executable stack. For backwards compatibility,
+    // Linux (the kernel when loading an executable, and glibc when loading a shared library)
+    // assumes you need an executable stack unless you request otherwise. In linked outputs
+    // (executables and shared libraries) this is in the program header: the flags of a
+    // PT_GNU_STACK entry specify stack permissions, and the default if unspecified is RWX. In
+    // intermediate objects (.o files) this is conveyed via the presence of an empty-length
+    // .note.GNU-stack, which is marked as an executable section (SHF_EXECINSTR) if the object
+    // needs an executable stack.
+    //
+    // For now we only check binaries because of an LLVM bug that causes .o files to be missing a
+    // .note.GNU-stack section, which we are overriding with -Wl,-z,noexecstack.
+
+    if matches!(elf.e_type(endian), ET_EXEC | ET_DYN) {
+        let mut found_pt_gnu_stack = false;
+        for phdr in elf.program_headers(endian, data)? {
+            if phdr.p_type(endian) != PT_GNU_STACK {
+                continue;
+            }
+            found_pt_gnu_stack = true;
+            if (phdr.p_flags(endian) & PF_X) != 0 {
+                context
+                    .errors
+                    .push(format!("{} requests executable stack", path.display()));
+            }
+        }
+        if !found_pt_gnu_stack {
+            context.errors.push(format!(
+                "{} missing PT_GNU_STACK header (defaults to executable stack)",
+                path.display(),
+            ));
+        }
+    }
+
     Ok(())
 }