linux-kernel - Re: [PATCH 02/13] hvc: Fix use of uninitialized array in udbg_hvc

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Date: Thu, 21 Dec 2023 23:09:49 +1100
From: Michael Ellerman <mpe@...erman.id.au>
To: Christophe Leroy <christophe.leroy@...roup.eu>, Nicholas Miehlbradt
 <nicholas@...ux.ibm.com>, "glider@...gle.com" <glider@...gle.com>,
 "elver@...gle.com" <elver@...gle.com>, "dvyukov@...gle.com"
 <dvyukov@...gle.com>, "akpm@...ux-foundation.org"
 <akpm@...ux-foundation.org>, "npiggin@...il.com" <npiggin@...il.com>
Cc: "linux-mm@...ck.org" <linux-mm@...ck.org>, "kasan-dev@...glegroups.com"
 <kasan-dev@...glegroups.com>, "iii@...ux.ibm.com" <iii@...ux.ibm.com>,
 "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 02/13] hvc: Fix use of uninitialized array in udbg_hvc_putc

Christophe Leroy <christophe.leroy@...roup.eu> writes:
> Le 14/12/2023 à 06:55, Nicholas Miehlbradt a écrit :
>> All elements of bounce_buffer are eventually read and passed to the
>> hypervisor so it should probably be fully initialized.
>
> should or shall ?
>
>> 
>> Signed-off-by: Nicholas Miehlbradt <nicholas@...ux.ibm.com>
>
> Should be a Fixed: tag ?
>
>> ---
>>   drivers/tty/hvc/hvc_vio.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/drivers/tty/hvc/hvc_vio.c b/drivers/tty/hvc/hvc_vio.c
>> index 736b230f5ec0..1e88bfcdde20 100644
>> --- a/drivers/tty/hvc/hvc_vio.c
>> +++ b/drivers/tty/hvc/hvc_vio.c
>> @@ -227,7 +227,7 @@ static const struct hv_ops hvterm_hvsi_ops = {
>>   static void udbg_hvc_putc(char c)
>>   {
>>   	int count = -1;
>> -	unsigned char bounce_buffer[16];
>> +	unsigned char bounce_buffer[16] = { 0 };
>
> Why 16 while we have a count of 1 in the call to hvterm_raw_put_chars() ?

Because hvterm_raw_put_chars() calls hvc_put_chars() which requires a 16
byte buffer, because it passes the buffer directly to firmware which
expects a 16 byte buffer.

It's a pretty horrible calling convention, but I guess it's to avoid
needing another bounce buffer inside hvc_put_chars().

We should probably do the change below, to at least document the
interface better.

cheers


diff --git a/arch/powerpc/include/asm/hvconsole.h b/arch/powerpc/include/asm/hvconsole.h
index ccb2034506f0..0ee7ed019e23 100644
--- a/arch/powerpc/include/asm/hvconsole.h
+++ b/arch/powerpc/include/asm/hvconsole.h
@@ -22,7 +22,7 @@
  * parm is included to conform to put_chars() function pointer template
  */
 extern int hvc_get_chars(uint32_t vtermno, char *buf, int count);
-extern int hvc_put_chars(uint32_t vtermno, const char *buf, int count);
+extern int hvc_put_chars(uint32_t vtermno, const char buf[16], int count);

 /* Provided by HVC VIO */
 void hvc_vio_init_early(void);
diff --git a/arch/powerpc/platforms/pseries/hvconsole.c b/arch/powerpc/platforms/pseries/hvconsole.c
index 1ac52963e08b..c40a82e49d59 100644
--- a/arch/powerpc/platforms/pseries/hvconsole.c
+++ b/arch/powerpc/platforms/pseries/hvconsole.c
@@ -52,7 +52,7 @@ EXPORT_SYMBOL(hvc_get_chars);
  *     firmware. Must be at least 16 bytes, even if count is less than 16.
  * @count: Send this number of characters.
  */
-int hvc_put_chars(uint32_t vtermno, const char *buf, int count)
+int hvc_put_chars(uint32_t vtermno, const char buf[16], int count)
 {
        unsigned long *lbuf = (unsigned long *) buf;
        long ret;
diff --git a/drivers/tty/hvc/hvc_vio.c b/drivers/tty/hvc/hvc_vio.c
index 736b230f5ec0..011b239a7e52 100644
--- a/drivers/tty/hvc/hvc_vio.c
+++ b/drivers/tty/hvc/hvc_vio.c
@@ -115,7 +115,7 @@ static int hvterm_raw_get_chars(uint32_t vtermno, char *buf, int count)
  *       you are sending fewer chars.
  * @count: number of chars to send.
  */
-static int hvterm_raw_put_chars(uint32_t vtermno, const char *buf, int count)
+static int hvterm_raw_put_chars(uint32_t vtermno, const char buf[16], int count)
 {
        struct hvterm_priv *pv = hvterm_privs[vtermno];