netdev - [RFC net-next] net: sysctl data could be in .bss

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Message-Id: <20211012155542.827631-1-atenart@kernel.org>
Date:   Tue, 12 Oct 2021 17:55:42 +0200
From:   Antoine Tenart <atenart@...nel.org>
To:     davem@...emloft.net, kuba@...nel.org
Cc:     Antoine Tenart <atenart@...nel.org>, jonathon.reinhart@...il.com,
        netdev@...r.kernel.org
Subject: [RFC net-next] net: sysctl data could be in .bss

A check is made when registering non-init netns sysctl files to ensure
their data pointer does not point to a global data section. This works
well for modules as the check is made against the whole module address
space (is_module_address). But when built-in, the check is made against
the .data section. However global variables initialized to 0 can be in
.bss (-fzero-initialized-in-bss).

Add an extra check to make sure the sysctl data does not point to the
.bss section either.

Signed-off-by: Antoine Tenart <atenart@...nel.org>
---

Hello,

This is sent as an RFC as I'd like a fix[1] to be merged before to
avoid introducing a new warning. But this can be reviewed in the
meantime.

I'm not sending this as a fix to avoid possible new warnings in stable
kernels. (The actual fixes of sysctl files should go).

I think this can go through the net-next tree as kernel/extable.c
doesn't seem to be under any subsystem and a conflict is unlikely to
happen.

Thanks!
Antoine

[1] https://lore.kernel.org/all/20211012145437.754391-1-atenart@kernel.org/T/

 include/linux/kernel.h | 1 +
 kernel/extable.c       | 8 ++++++++
 net/sysctl_net.c       | 2 +-
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 2776423a587e..beb61d0ab220 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -231,6 +231,7 @@ extern char *next_arg(char *args, char **param, char **val);
 extern int core_kernel_text(unsigned long addr);
 extern int init_kernel_text(unsigned long addr);
 extern int core_kernel_data(unsigned long addr);
+extern int core_kernel_bss(unsigned long addr);
 extern int __kernel_text_address(unsigned long addr);
 extern int kernel_text_address(unsigned long addr);
 extern int func_ptr_is_kernel_text(void *ptr);
diff --git a/kernel/extable.c b/kernel/extable.c
index b0ea5eb0c3b4..477a4b6c8f63 100644
--- a/kernel/extable.c
+++ b/kernel/extable.c
@@ -100,6 +100,14 @@ int core_kernel_data(unsigned long addr)
 	return 0;
 }
 
+int core_kernel_bss(unsigned long addr)
+{
+	if (addr >= (unsigned long)__bss_start &&
+	    addr < (unsigned long)__bss_stop)
+		return 1;
+	return 0;
+}
+
 int __kernel_text_address(unsigned long addr)
 {
 	if (kernel_text_address(addr))
diff --git a/net/sysctl_net.c b/net/sysctl_net.c
index f6cb0d4d114c..d883cf65029f 100644
--- a/net/sysctl_net.c
+++ b/net/sysctl_net.c
@@ -144,7 +144,7 @@ static void ensure_safe_net_sysctl(struct net *net, const char *path,
 		addr = (unsigned long)ent->data;
 		if (is_module_address(addr))
 			where = "module";
-		else if (core_kernel_data(addr))
+		else if (core_kernel_data(addr) || core_kernel_bss(addr))
 			where = "kernel";
 		else
 			continue;
-- 
2.31.1