ndr: avoid excessive reallocing in pull_string_array

Before, talloc_realloc() was being called n times for an array of
length n. This could be very expensive on long string arrays since it
is reasonable to assume each realloc moves O(n) bytes.

This addresses at least one OSS-Fuzz bug, making a timing out test case
100 times faster. Credit to OSS-Fuzz.

REF: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=19706

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Noel Power <npower@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

1 files changed, 60 insertions, 11 deletions

diff --git a/librpc/ndr/ndr_string.c b/librpc/ndr/ndr_string.c
index 4fd2006be20..bddab9edd51 100644
--- a/librpc/ndr/ndr_string.c
+++ b/librpc/ndr/ndr_string.c
@@ -348,6 +348,44 @@ _PUBLIC_ uint32_t ndr_size_string(int ret, const char * const* string, int flags
 	return ret+strlen(*string)+1;
 }
 
+static uint32_t guess_string_array_size(struct ndr_pull *ndr, int ndr_flags)
+{
+	/*
+	 * Here we could do something clever like count the number of zeros in
+	 * the ndr data, but it is probably sufficient to pick a lowish number
+	 * (compared to the overhead of the talloc header) and let the
+	 * expontential resizing deal with longer arrays.
+	 */
+	return 5;
+}
+
+static enum ndr_err_code extend_string_array(struct ndr_pull *ndr,
+					     const char ***_a,
+					     uint32_t *count)
+{
+	const char **a = *_a;
+	uint32_t inc = *count / 4 + 3;
+	uint32_t alloc_size = *count + inc;
+
+	if (alloc_size < *count) {
+		/* overflow ! */
+		return NDR_ERR_ALLOC;
+	}
+	/*
+	 * We allocate and zero two more bytes than we report back, so that
+	 * the string array will always be NULL terminated.
+	 */
+	a = talloc_realloc(ndr->current_mem_ctx, a,
+			   const char *,
+			   alloc_size);
+	NDR_ERR_HAVE_NO_MEMORY(a);
+
+	memset(a + *count, 0, inc * sizeof(a[0]));
+	*_a = a;
+	*count = alloc_size - 2;
+	return NDR_ERR_SUCCESS;
+}
+
 /**
   pull a general string array from the wire
 */
@@ -357,14 +395,19 @@ _PUBLIC_ enum ndr_err_code ndr_pull_string_array(struct ndr_pull *ndr, int ndr_f
 	uint32_t count;
 	unsigned flags = ndr->flags;
 	unsigned saved_flags = ndr->flags;
+	uint32_t alloc_size;
 
 	if (!(ndr_flags & NDR_SCALARS)) {
 		return NDR_ERR_SUCCESS;
 	}
 
+	alloc_size = guess_string_array_size(ndr, ndr_flags);
+	a = talloc_zero_array(ndr->current_mem_ctx, const char *, alloc_size + 2);
+	NDR_ERR_HAVE_NO_MEMORY(a);
+
 	switch (flags & (LIBNDR_FLAG_STR_NULLTERM|LIBNDR_FLAG_STR_NOTERM)) {
 	case LIBNDR_FLAG_STR_NULLTERM:
-		/* 
+		/*
 		 * here the strings are null terminated
 		 * but also the array is null terminated if LIBNDR_FLAG_REMAINING
 		 * is specified
@@ -372,10 +415,11 @@ _PUBLIC_ enum ndr_err_code ndr_pull_string_array(struct ndr_pull *ndr, int ndr_f
 		for (count = 0;; count++) {
 			TALLOC_CTX *tmp_ctx;
 			const char *s = NULL;
-			a = talloc_realloc(ndr->current_mem_ctx, a, const char *, count + 2);
-			NDR_ERR_HAVE_NO_MEMORY(a);
-			a[count]   = NULL;
-			a[count+1]   = NULL;
+			if (count == alloc_size) {
+				NDR_CHECK(extend_string_array(ndr,
+							      &a,
+							      &alloc_size));
+			}
 
 			tmp_ctx = ndr->current_mem_ctx;
 			ndr->current_mem_ctx = a;
@@ -393,6 +437,8 @@ _PUBLIC_ enum ndr_err_code ndr_pull_string_array(struct ndr_pull *ndr, int ndr_f
 				a[count] = s;
 			}
 		}
+		a = talloc_realloc(ndr->current_mem_ctx, a, const char *, count + 1);
+		NDR_ERR_HAVE_NO_MEMORY(a);
 
 		*_a =a;
 		break;
@@ -404,7 +450,7 @@ _PUBLIC_ enum ndr_err_code ndr_pull_string_array(struct ndr_pull *ndr, int ndr_f
 		}
 		/*
 		 * here the strings are not null terminated
-		 * but serarated by a null terminator
+		 * but separated by a null terminator
 		 *
 		 * which means the same as:
 		 * Every string is null terminated exept the last
@@ -422,10 +468,11 @@ _PUBLIC_ enum ndr_err_code ndr_pull_string_array(struct ndr_pull *ndr, int ndr_f
 		for (count = 0; ((ndr->data_size - ndr->offset) > 0); count++) {
 			TALLOC_CTX *tmp_ctx;
 			const char *s = NULL;
-			a = talloc_realloc(ndr->current_mem_ctx, a, const char *, count + 2);
-			NDR_ERR_HAVE_NO_MEMORY(a);
-			a[count]   = NULL;
-			a[count+1]   = NULL;
+			if (count == alloc_size) {
+				NDR_CHECK(extend_string_array(ndr,
+							      &a,
+							      &alloc_size));
+			}
 
 			tmp_ctx = ndr->current_mem_ctx;
 			ndr->current_mem_ctx = a;
@@ -434,7 +481,9 @@ _PUBLIC_ enum ndr_err_code ndr_pull_string_array(struct ndr_pull *ndr, int ndr_f
 			a[count] = s;
 		}
 
-		*_a =a;
+		a = talloc_realloc(ndr->current_mem_ctx, a, const char *, count + 1);
+		NDR_ERR_HAVE_NO_MEMORY(a);
+		*_a = a;
 		break;
 
 	default: