efi/x86: Remove extra headroom for setup block

The following commit:

  223e3ee5 ("efi/x86: add headroom to decompressor BSS to account for setup block")

added headroom to the PE image to account for the setup block, which
wasn't used for the decompression buffer.

Now that the decompression buffer is located at the start of the image,
and includes the setup block, this is no longer required.

Add a check to make sure that the head section of the compressed kernel
won't overwrite itself while relocating. This is only for
future-proofing as with current limits on the setup and the actual size
of the head section, this can never happen.
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200303221205.4048668-5-nivedita@alum.mit.edu
Link: https://lore.kernel.org/r/20200308080859.21568-19-ardb@kernel.org

arch/x86/boot/tools/build.c

@@ -65,6 +65,8 @@ unsigned long efi_pe_entry;
 unsigned long efi32_pe_entry;
 unsigned long kernel_info;
 unsigned long startup_64;
+unsigned long _ehead;
+unsigned long _end;
 
 /*----------------------------------------------------------------------*/
 
@@ -232,7 +234,7 @@ static void update_pecoff_text(unsigned int text_start, unsigned int file_sz,
 {
 	unsigned int pe_header;
 	unsigned int text_sz = file_sz - text_start;
-	unsigned int bss_sz = init_sz + text_start - file_sz;
+	unsigned int bss_sz = init_sz - file_sz;
 
 	pe_header = get_unaligned_le32(&buf[0x3c]);
 
@@ -259,7 +261,7 @@ static void update_pecoff_text(unsigned int text_start, unsigned int file_sz,
 	put_unaligned_le32(file_sz - 512 + bss_sz, &buf[pe_header + 0x1c]);
 
 	/* Size of image */
-	put_unaligned_le32(init_sz + text_start, &buf[pe_header + 0x50]);
+	put_unaligned_le32(init_sz, &buf[pe_header + 0x50]);
 
 	/*
 	 * Address of entry point for PE/COFF executable
@@ -360,6 +362,8 @@ static void parse_zoffset(char *fname)
 		PARSE_ZOFS(p, efi32_pe_entry);
 		PARSE_ZOFS(p, kernel_info);
 		PARSE_ZOFS(p, startup_64);
+		PARSE_ZOFS(p, _ehead);
+		PARSE_ZOFS(p, _end);
 
 		p = strchr(p, '\n');
 		while (p && (*p == '\r' || *p == '\n'))
@@ -444,6 +448,26 @@ int main(int argc, char ** argv)
 	put_unaligned_le32(sys_size, &buf[0x1f4]);
 
 	init_sz = get_unaligned_le32(&buf[0x260]);
+#ifdef CONFIG_EFI_STUB
+	/*
+	 * The decompression buffer will start at ImageBase. When relocating
+	 * the compressed kernel to its end, we must ensure that the head
+	 * section does not get overwritten.  The head section occupies
+	 * [i, i + _ehead), and the destination is [init_sz - _end, init_sz).
+	 *
+	 * At present these should never overlap, because 'i' is at most 32k
+	 * because of SETUP_SECT_MAX, '_ehead' is less than 1k, and the
+	 * calculation of INIT_SIZE in boot/header.S ensures that
+	 * 'init_sz - _end' is at least 64k.
+	 *
+	 * For future-proofing, increase init_sz if necessary.
+	 */
+
+	if (init_sz - _end < i + _ehead) {
+		init_sz = (i + _ehead + _end + 4095) & ~4095;
+		put_unaligned_le32(init_sz, &buf[0x260]);
+	}
+#endif
 	update_pecoff_text(setup_sectors * 512, i + (sys_size * 16), init_sz);
 
 	efi_stub_entry_update();