[PATCH] binfmt_flat: revert "binfmt_flat: don't offset the data start"

[Max Filippov <jcmvbkbc@gmail.com>]

binfmt_flat loader uses the gap between text and data to store data
segment pointers for the libraries. Even in the absence of shared
libraries it stores at least one pointer to the executable's own data
segment. Text and data can go back to back in the flat binary image and
without offsetting data segment last few instructions in the text
segment may get corrupted by the data segment pointer.

Fix it by reverting commit a2357223c50a ("binfmt_flat: don't offset the
data start").

Cc: stable@vger.kernel.org
Fixes: a2357223c50a ("binfmt_flat: don't offset the data start")
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
---
 fs/binfmt_flat.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/fs/binfmt_flat.c b/fs/binfmt_flat.c
index f2f9086ebe98..b9c658e0548e 100644
--- a/fs/binfmt_flat.c
+++ b/fs/binfmt_flat.c
@@ -576,7 +576,7 @@ static int load_flat_file(struct linux_binprm *bprm,
 			goto err;
 		}
 
-		len = data_len + extra;
+		len = data_len + extra + MAX_SHARED_LIBS * sizeof(unsigned long);
 		len = PAGE_ALIGN(len);
 		realdatastart = vm_mmap(NULL, 0, len,
 			PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE, 0);
@@ -590,7 +590,9 @@ static int load_flat_file(struct linux_binprm *bprm,
 			vm_munmap(textpos, text_len);
 			goto err;
 		}
-		datapos = ALIGN(realdatastart, FLAT_DATA_ALIGN);
+		datapos = ALIGN(realdatastart +
+				MAX_SHARED_LIBS * sizeof(unsigned long),
+				FLAT_DATA_ALIGN);
 
 		pr_debug("Allocated data+bss+stack (%u bytes): %lx\n",
 			 data_len + bss_len + stack_len, datapos);
@@ -620,7 +622,7 @@ static int load_flat_file(struct linux_binprm *bprm,
 		memp_size = len;
 	} else {
 
-		len = text_len + data_len + extra;
+		len = text_len + data_len + extra + MAX_SHARED_LIBS * sizeof(u32);
 		len = PAGE_ALIGN(len);
 		textpos = vm_mmap(NULL, 0, len,
 			PROT_READ | PROT_EXEC | PROT_WRITE, MAP_PRIVATE, 0);
@@ -635,7 +637,9 @@ static int load_flat_file(struct linux_binprm *bprm,
 		}
 
 		realdatastart = textpos + ntohl(hdr->data_start);
-		datapos = ALIGN(realdatastart, FLAT_DATA_ALIGN);
+		datapos = ALIGN(realdatastart +
+				MAX_SHARED_LIBS * sizeof(u32),
+				FLAT_DATA_ALIGN);
 
 		reloc = (__be32 __user *)
 			(datapos + (ntohl(hdr->reloc_start) - text_len));
@@ -652,9 +656,8 @@ static int load_flat_file(struct linux_binprm *bprm,
 					 (text_len + full_data
 						  - sizeof(struct flat_hdr)),
 					 0);
-			if (datapos != realdatastart)
-				memmove((void *)datapos, (void *)realdatastart,
-						full_data);
+			memmove((void *) datapos, (void *) realdatastart,
+					full_data);
 #else
 			/*
 			 * This is used on MMU systems mainly for testing.
@@ -710,7 +713,8 @@ static int load_flat_file(struct linux_binprm *bprm,
 		if (IS_ERR_VALUE(result)) {
 			ret = result;
 			pr_err("Unable to read code+data+bss, errno %d\n", ret);
-			vm_munmap(textpos, text_len + data_len + extra);
+			vm_munmap(textpos, text_len + data_len + extra +
+				MAX_SHARED_LIBS * sizeof(u32));
 			goto err;
 		}
 	}
-- 
2.20.1