[PATCH] ARM: zImage: Fix stack overflow in merge_fdt

Discussion:

[PATCH] ARM: zImage: Fix stack overflow in merge_fdt_bootargs()

Rask Ingemann Lambertsen

2017-07-16 21:43:15 UTC

This function is called very early on from head.S and currently sets up a
stack frame of more than 1024 bytes:

atags_to_fdt.c: In function ‘merge_fdt_bootargs’:
atags_to_fdt.c:98:1: warning: the frame size of 1032 bytes is larger than 1024 bytes [-Wframe-larger-than=]

This causes a crash and failure to boot with some combinations of kernel
version, gcc version and dtb, such as kernel version 4.1-rc1 of 4.1.0,
gcc version 5.4.1 20161019 (Debian 5.4.1-3) and tegra20-trimslice.dtb.

With this patch, merge_fdt_bootargs() is rewritten to not use a large buffer,
thereby solving the problem of the stack overflow.

As a side effect of this rewrite, you no longer get a space added in front
of the kernel command line when no bootargs property was found in the fdt.

Signed-off-by: Rask Ingemann Lambertsen <***@formelder.dk>
Fixes: d0f34a11ddab ("ARM: 7437/1: zImage: Allow DTB command line concatenation with ATAG_CMDLINE")
---

I have tested that this works properly when
a) only the FDT bootargs are provided,
b) only the ATAG command line is provided, and
c) both the FDT bootargs and the ATAG command line are provided.

arch/arm/boot/compressed/atags_to_fdt.c | 59 +++++++++++++++++++--------------
1 file changed, 35 insertions(+), 24 deletions(-)

diff --git a/arch/arm/boot/compressed/atags_to_fdt.c b/arch/arm/boot/compressed/atags_to_fdt.c
index 9448aa0c6686..ede23ef2e889 100644
--- a/arch/arm/boot/compressed/atags_to_fdt.c
+++ b/arch/arm/boot/compressed/atags_to_fdt.c
@@ -55,6 +55,27 @@ static const void *getprop(const void *fdt, const char *node_path,
return fdt_getprop(fdt, offset, property, len);
}

+static void *getprop_w(void *fdt, const char *node_path,
+ const char *property, int *len)
+{
+ int offset = fdt_path_offset(fdt, node_path);
+
+ if (offset == -FDT_ERR_NOTFOUND)
+ return NULL;
+
+ return fdt_getprop_w(fdt, offset, property, len);
+}
+
+static int appendprop_string(void *fdt, const char *node_path,
+ const char *property, const char *string)
+{
+ int offset = node_offset(fdt, node_path);
+
+ if (offset < 0)
+ return offset;
+ return fdt_appendprop_string(fdt, offset, property, string);
+}
+
static uint32_t get_cell_size(const void *fdt)
{
int len;
@@ -66,35 +87,25 @@ static uint32_t get_cell_size(const void *fdt)
return cell_size;
}

-static void merge_fdt_bootargs(void *fdt, const char *fdt_cmdline)
-{
- char cmdline[COMMAND_LINE_SIZE];
- const char *fdt_bootargs;
- char *ptr = cmdline;
- int len = 0;
-
- /* copy the fdt command line into the buffer */
- fdt_bootargs = getprop(fdt, "/chosen", "bootargs", &len);
- if (fdt_bootargs)
- if (len < COMMAND_LINE_SIZE) {
- memcpy(ptr, fdt_bootargs, len);
- /* len is the length of the string
- * including the NULL terminator */
- ptr += len - 1;
- }
-
- /* and append the ATAG_CMDLINE */
- if (fdt_cmdline) {
- len = strlen(fdt_cmdline);
- if (ptr - cmdline + len + 2 < COMMAND_LINE_SIZE) {
- *ptr++ = ' ';
- memcpy(ptr, fdt_cmdline, len);
- ptr += len;
- }
- }
- *ptr = '\0';
-
- setprop_string(fdt, "/chosen", "bootargs", cmdline);
+/* This is called early on from head.S, so it can't use much stack. */
+static void merge_fdt_bootargs(void *fdt, const char *atag_cmdline)
+{
+ char *fdt_bootargs;
+ int len = 0;
+
+ /* With no ATAG command line or an empty one, there is nothing to do. */
+ if (!atag_cmdline || strlen(atag_cmdline) == 0)
+ return;
+
+ fdt_bootargs = getprop_w(fdt, "/chosen", "bootargs", &len);
+
+ /* With no FDT command line or an empty one, just use the ATAG one. */
+ if (!fdt_bootargs || len <= 1) {
+ setprop_string(fdt, "/chosen", "bootargs", atag_cmdline);
+ return;
+ }
+ fdt_bootargs[len - 1] = ' ';
+ appendprop_string(fdt, "/chosen", "bootargs", atag_cmdline);
}

/*

--
2.13.2

Richard Genoud

2017-07-18 07:39:10 UTC

Permalink

Post by Rask Ingemann Lambertsen
This function is called very early on from head.S and currently sets up a
atags_to_fdt.c:98:1: warning: the frame size of 1032 bytes is larger than 1024 bytes [-Wframe-larger-than=]
This causes a crash and failure to boot with some combinations of kernel
version, gcc version and dtb, such as kernel version 4.1-rc1 of 4.1.0,
gcc version 5.4.1 20161019 (Debian 5.4.1-3) and tegra20-trimslice.dtb.
With this patch, merge_fdt_bootargs() is rewritten to not use a large buffer,
thereby solving the problem of the stack overflow.
As a side effect of this rewrite, you no longer get a space added in front
of the kernel command line when no bootargs property was found in the fdt.
Fixes: d0f34a11ddab ("ARM: 7437/1: zImage: Allow DTB command line concatenation with ATAG_CMDLINE")
---
I have tested that this works properly when
a) only the FDT bootargs are provided,
b) only the ATAG command line is provided, and
c) both the FDT bootargs and the ATAG command line are provided.
arch/arm/boot/compressed/atags_to_fdt.c | 59 +++++++++++++++++++--------------
1 file changed, 35 insertions(+), 24 deletions(-)

Thanks for fixing that !

Post by Rask Ingemann Lambertsen
diff --git a/arch/arm/boot/compressed/atags_to_fdt.c b/arch/arm/boot/compressed/atags_to_fdt.c
index 9448aa0c6686..ede23ef2e889 100644
--- a/arch/arm/boot/compressed/atags_to_fdt.c
+++ b/arch/arm/boot/compressed/atags_to_fdt.c
@@ -55,6 +55,27 @@ static const void *getprop(const void *fdt, const char *node_path,
return fdt_getprop(fdt, offset, property, len);
}
+static void *getprop_w(void *fdt, const char *node_path,
+ const char *property, int *len)
+{
+ int offset = fdt_path_offset(fdt, node_path);
+
+ if (offset == -FDT_ERR_NOTFOUND)
+ return NULL;
+
+ return fdt_getprop_w(fdt, offset, property, len);
+}
+
+static int appendprop_string(void *fdt, const char *node_path,
+ const char *property, const char *string)
+{
+ int offset = node_offset(fdt, node_path);
+
+ if (offset < 0)
+ return offset;
+ return fdt_appendprop_string(fdt, offset, property, string);
+}
+
static uint32_t get_cell_size(const void *fdt)
{
int len;
@@ -66,35 +87,25 @@ static uint32_t get_cell_size(const void *fdt)
return cell_size;
}
-static void merge_fdt_bootargs(void *fdt, const char *fdt_cmdline)
-{
- char cmdline[COMMAND_LINE_SIZE];
- const char *fdt_bootargs;
- char *ptr = cmdline;
- int len = 0;
-
- /* copy the fdt command line into the buffer */
- fdt_bootargs = getprop(fdt, "/chosen", "bootargs", &len);
- if (fdt_bootargs)
- if (len < COMMAND_LINE_SIZE) {
- memcpy(ptr, fdt_bootargs, len);
- /* len is the length of the string
- * including the NULL terminator */
- ptr += len - 1;
- }
-
- /* and append the ATAG_CMDLINE */
- if (fdt_cmdline) {
- len = strlen(fdt_cmdline);
- if (ptr - cmdline + len + 2 < COMMAND_LINE_SIZE) {
- *ptr++ = ' ';
- memcpy(ptr, fdt_cmdline, len);
- ptr += len;
- }
- }
- *ptr = '\0';
-
- setprop_string(fdt, "/chosen", "bootargs", cmdline);
+/* This is called early on from head.S, so it can't use much stack. */
+static void merge_fdt_bootargs(void *fdt, const char *atag_cmdline)
+{
+ char *fdt_bootargs;
+ int len = 0;
+
+ /* With no ATAG command line or an empty one, there is nothing to do. */
+ if (!atag_cmdline || strlen(atag_cmdline) == 0)
+ return;
+
+ fdt_bootargs = getprop_w(fdt, "/chosen", "bootargs", &len);
+
+ /* With no FDT command line or an empty one, just use the ATAG one. */
+ if (!fdt_bootargs || len <= 1) {
+ setprop_string(fdt, "/chosen", "bootargs", atag_cmdline);
+ return;
+ }
+ fdt_bootargs[len - 1] = ' ';
+ appendprop_string(fdt, "/chosen", "bootargs", atag_cmdline);

Let's say appendprop_string() fails for whatever reason, the /chosen
string won't be null terminated anymore.
Won't it cause some problems ?

Post by Rask Ingemann Lambertsen
}
/*

Richard.

Rask Ingemann Lambertsen

2017-07-18 21:40:22 UTC

Permalink

[snip]

Post by Richard Genoud

Post by Rask Ingemann Lambertsen
+/* This is called early on from head.S, so it can't use much stack. */
+static void merge_fdt_bootargs(void *fdt, const char *atag_cmdline)
+{
+ char *fdt_bootargs;
+ int len = 0;
+
+ /* With no ATAG command line or an empty one, there is nothing to do. */
+ if (!atag_cmdline || strlen(atag_cmdline) == 0)
+ return;
+
+ fdt_bootargs = getprop_w(fdt, "/chosen", "bootargs", &len);
+
+ /* With no FDT command line or an empty one, just use the ATAG one. */
+ if (!fdt_bootargs || len <= 1) {
+ setprop_string(fdt, "/chosen", "bootargs", atag_cmdline);
+ return;
+ }
+ fdt_bootargs[len - 1] = ' ';
+ appendprop_string(fdt, "/chosen", "bootargs", atag_cmdline);

Let's say appendprop_string() fails for whatever reason, the /chosen
string won't be null terminated anymore.

Good catch, I will fix that. Thank you for your review.

Post by Richard Genoud
Won't it cause some problems ?

It's OK at the moment because the last character of the "bootargs" property
is not used. This is the code from early_init_dt_scan_chosen() in
drivers/of/fdt.c:

/* Retrieve command line */
p = of_get_flat_dt_prop(node, "bootargs", &l);
if (p != NULL && l > 0)
strlcpy(data, p, min((int)l, COMMAND_LINE_SIZE));

But not all accesses are that careful. I found get_cmdline() in
arch/arm64/kernel/kaslr.c using

prop = fdt_getprop(fdt, node, "bootargs", NULL);

so there is an expectation that the string is NUL terminated.

--
Rask Ingemann Lambertsen

Pavel Machek

2017-07-19 08:15:36 UTC

Permalink

Hi!

I tested that it does not break boot on N900. I hoped that it would
fix boot on N950 (but no luck there so far).

Tested-by: Pavel Machek <***@ucw.cz>

AFAICT this is regression in v4.13-rc1. Thus it is quite surprising
that there are no comments here.

Genoud? Russell?

Pavel

Post by Rask Ingemann Lambertsen
I have tested that this works properly when
a) only the FDT bootargs are provided,
b) only the ATAG command line is provided, and
c) both the FDT bootargs and the ATAG command line are provided.
arch/arm/boot/compressed/atags_to_fdt.c | 59 +++++++++++++++++++--------------
1 file changed, 35 insertions(+), 24 deletions(-)
diff --git a/arch/arm/boot/compressed/atags_to_fdt.c b/arch/arm/boot/compressed/atags_to_fdt.c
index 9448aa0c6686..ede23ef2e889 100644
--- a/arch/arm/boot/compressed/atags_to_fdt.c
+++ b/arch/arm/boot/compressed/atags_to_fdt.c
@@ -55,6 +55,27 @@ static const void *getprop(const void *fdt, const char *node_path,
return fdt_getprop(fdt, offset, property, len);
}
+static void *getprop_w(void *fdt, const char *node_path,
+ const char *property, int *len)
+{
+ int offset = fdt_path_offset(fdt, node_path);
+
+ if (offset == -FDT_ERR_NOTFOUND)
+ return NULL;
+
+ return fdt_getprop_w(fdt, offset, property, len);
+}
+
+static int appendprop_string(void *fdt, const char *node_path,
+ const char *property, const char *string)
+{
+ int offset = node_offset(fdt, node_path);
+
+ if (offset < 0)
+ return offset;
+ return fdt_appendprop_string(fdt, offset, property, string);
+}
+
static uint32_t get_cell_size(const void *fdt)
{
int len;
@@ -66,35 +87,25 @@ static uint32_t get_cell_size(const void *fdt)
return cell_size;
}
-static void merge_fdt_bootargs(void *fdt, const char *fdt_cmdline)
-{
- char cmdline[COMMAND_LINE_SIZE];
- const char *fdt_bootargs;
- char *ptr = cmdline;
- int len = 0;
-
- /* copy the fdt command line into the buffer */
- fdt_bootargs = getprop(fdt, "/chosen", "bootargs", &len);
- if (fdt_bootargs)
- if (len < COMMAND_LINE_SIZE) {
- memcpy(ptr, fdt_bootargs, len);
- /* len is the length of the string
- * including the NULL terminator */
- ptr += len - 1;
- }
-
- /* and append the ATAG_CMDLINE */
- if (fdt_cmdline) {
- len = strlen(fdt_cmdline);
- if (ptr - cmdline + len + 2 < COMMAND_LINE_SIZE) {
- *ptr++ = ' ';
- memcpy(ptr, fdt_cmdline, len);
- ptr += len;
- }
- }
- *ptr = '\0';
-
- setprop_string(fdt, "/chosen", "bootargs", cmdline);
+/* This is called early on from head.S, so it can't use much stack. */
+static void merge_fdt_bootargs(void *fdt, const char *atag_cmdline)
+{
+ char *fdt_bootargs;
+ int len = 0;
+
+ /* With no ATAG command line or an empty one, there is nothing to do. */
+ if (!atag_cmdline || strlen(atag_cmdline) == 0)
+ return;
+
+ fdt_bootargs = getprop_w(fdt, "/chosen", "bootargs", &len);
+
+ /* With no FDT command line or an empty one, just use the ATAG one. */
+ if (!fdt_bootargs || len <= 1) {
+ setprop_string(fdt, "/chosen", "bootargs", atag_cmdline);
+ return;
+ }
+ fdt_bootargs[len - 1] = ' ';
+ appendprop_string(fdt, "/chosen", "bootargs", atag_cmdline);
}
/*

--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

Sebastian Reichel

2017-07-19 08:47:27 UTC

Permalink

Hi,

Post by Pavel Machek

I tested that it does not break boot on N900. I hoped that it would
fix boot on N950 (but no luck there so far).
AFAICT this is regression in v4.13-rc1. Thus it is quite surprising
that there are no comments here.

No, it's not. I definitely saw the warning before (and see it now)
and images can be booted on all of the phones I'm working on (i.e.
N900, N950, Droid 4).

Anyawys thanks for fixing this properly. I will add the patch to my
dev branch and give it a test during the day.

-- Sebastian

Post by Pavel Machek
Genoud? Russell?
Pavel

--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

Sebastian Reichel

2017-07-19 09:12:51 UTC

Permalink

Hi,

Post by Sebastian Reichel

Post by Pavel Machek

No, it's not. I definitely saw the warning before (and see it now)
and images can be booted on all of the phones I'm working on (i.e.
N900, N950, Droid 4).
Anyawys thanks for fixing this properly. I will add the patch to my
dev branch and give it a test during the day.

It does work for me.

Tested-by: Sebastian Reichel <***@collabora.co.uk>

-- Sebastian