LinuxLists.cc - [PATCH v2] Drivers: hv: vmbus: Split memcpy of flex-array

2022-09-27 21:28:41

Subject: [PATCH v2] Drivers: hv: vmbus: Split memcpy of flex-array

To work around a misbehavior of the compiler's ability to see into
composite flexible array structs (as detailed in the coming memcpy()
hardening series[1]), split the memcpy() of the header and the payload
so no false positive run-time overflow warning will be generated. This
results in the already inlined memcpy getting unrolled a little more,
which very slightly increases text size:

$ size drivers/hv/vmbus_drv.o.before drivers/hv/vmbus_drv.o
text data bss dec hex filename
22968 5239 232 28439 6f17 drivers/hv/vmbus_drv.o.before
23032 5239 232 28503 6f57 drivers/hv/vmbus_drv.o

Avoids the run-time false-positive warning:

memcpy: detected field-spanning write (size 212) of single field "&ctx->msg" at drivers/hv/vmbus_drv.c:1133 (size 16)

[1] https://lore.kernel.org/linux-hardening/[email protected]/

Cc: "K. Y. Srinivasan" <[email protected]>
Cc: Haiyang Zhang <[email protected]>
Cc: Stephen Hemminger <[email protected]>
Cc: Wei Liu <[email protected]>
Cc: Dexuan Cui <[email protected]>
Cc: [email protected]
Reported-by: Nathan Chancellor <[email protected]>
Reported-by: "Gustavo A. R. Silva" <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
---
v2: - fix commit log, add tags and exact warning test from Nathan
v1: https://lore.kernel.org/lkml/[email protected]
---
drivers/hv/vmbus_drv.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index 23c680d1a0f5..9b111a8262e3 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -1131,7 +1131,8 @@ void vmbus_on_msg_dpc(unsigned long data)
return;

INIT_WORK(&ctx->work, vmbus_onmessage_work);
- memcpy(&ctx->msg, &msg_copy, sizeof(msg->header) + payload_size);
+ ctx->msg.header = msg_copy.header;
+ memcpy(&ctx->msg.payload, msg_copy.u.payload, payload_size);

/*
* The host can generate a rescind message while we
--
2.34.1

2022-09-27 21:30:18

by Michael Kelley (LINUX)

[permalink] [raw]

Subject: RE: [PATCH v2] Drivers: hv: vmbus: Split memcpy of flex-array

From: Kees Cook <[email protected]> Sent: Tuesday, September 27, 2022 2:18 PM
>
> To work around a misbehavior of the compiler's ability to see into
> composite flexible array structs (as detailed in the coming memcpy()
> hardening series[1]), split the memcpy() of the header and the payload
> so no false positive run-time overflow warning will be generated. This
> results in the already inlined memcpy getting unrolled a little more,
> which very slightly increases text size:
>
> $ size drivers/hv/vmbus_drv.o.before drivers/hv/vmbus_drv.o
> text data bss dec hex filename
> 22968 5239 232 28439 6f17 drivers/hv/vmbus_drv.o.before
> 23032 5239 232 28503 6f57 drivers/hv/vmbus_drv.o
>
> Avoids the run-time false-positive warning:
>
> memcpy: detected field-spanning write (size 212) of single field "&ctx->msg" at
> drivers/hv/vmbus_drv.c:1133 (size 16)
>
> [1] https://lore.kernel.org/linux-hardening/[email protected]/
>
> Cc: "K. Y. Srinivasan" <[email protected]>
> Cc: Haiyang Zhang <[email protected]>
> Cc: Stephen Hemminger <[email protected]>
> Cc: Wei Liu <[email protected]>
> Cc: Dexuan Cui <[email protected]>
> Cc: [email protected]
> Reported-by: Nathan Chancellor <[email protected]>
> Reported-by: "Gustavo A. R. Silva" <[email protected]>
> Tested-by: Nathan Chancellor <[email protected]>
> Signed-off-by: Kees Cook <[email protected]>
> ---
> v2: - fix commit log, add tags and exact warning test from Nathan
> v1: https://lore.kernel.org/lkml/[email protected]/
> ---
> drivers/hv/vmbus_drv.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 23c680d1a0f5..9b111a8262e3 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -1131,7 +1131,8 @@ void vmbus_on_msg_dpc(unsigned long data)
> return;
>
> INIT_WORK(&ctx->work, vmbus_onmessage_work);
> - memcpy(&ctx->msg, &msg_copy, sizeof(msg->header) + payload_size);
> + ctx->msg.header = msg_copy.header;
> + memcpy(&ctx->msg.payload, msg_copy.u.payload, payload_size);
>
> /*
> * The host can generate a rescind message while we
> --
> 2.34.1

Reviewed-by: Michael Kelley <[email protected]>

2022-09-27 21:31:16

by Gustavo A. R. Silva

[permalink] [raw]

Subject: Re: [PATCH v2] Drivers: hv: vmbus: Split memcpy of flex-array

On Tue, Sep 27, 2022 at 02:17:36PM -0700, Kees Cook wrote:
> To work around a misbehavior of the compiler's ability to see into
> composite flexible array structs (as detailed in the coming memcpy()
> hardening series[1]), split the memcpy() of the header and the payload
> so no false positive run-time overflow warning will be generated. This
> results in the already inlined memcpy getting unrolled a little more,
> which very slightly increases text size:
>
> $ size drivers/hv/vmbus_drv.o.before drivers/hv/vmbus_drv.o
> text data bss dec hex filename
> 22968 5239 232 28439 6f17 drivers/hv/vmbus_drv.o.before
> 23032 5239 232 28503 6f57 drivers/hv/vmbus_drv.o
>
> Avoids the run-time false-positive warning:
>
> memcpy: detected field-spanning write (size 212) of single field "&ctx->msg" at drivers/hv/vmbus_drv.c:1133 (size 16)
>
> [1] https://lore.kernel.org/linux-hardening/[email protected]/
>
> Cc: "K. Y. Srinivasan" <[email protected]>
> Cc: Haiyang Zhang <[email protected]>
> Cc: Stephen Hemminger <[email protected]>
> Cc: Wei Liu <[email protected]>
> Cc: Dexuan Cui <[email protected]>
> Cc: [email protected]
> Reported-by: Nathan Chancellor <[email protected]>
> Reported-by: "Gustavo A. R. Silva" <[email protected]>
> Tested-by: Nathan Chancellor <[email protected]>
> Signed-off-by: Kees Cook <[email protected]>

Reviewed-by: Gustavo A. R. Silva <[email protected]>

Thanks!
--
Gustavo

> ---
> v2: - fix commit log, add tags and exact warning test from Nathan
> v1: https://lore.kernel.org/lkml/[email protected]
> ---
> drivers/hv/vmbus_drv.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 23c680d1a0f5..9b111a8262e3 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -1131,7 +1131,8 @@ void vmbus_on_msg_dpc(unsigned long data)
> return;
>
> INIT_WORK(&ctx->work, vmbus_onmessage_work);
> - memcpy(&ctx->msg, &msg_copy, sizeof(msg->header) + payload_size);
> + ctx->msg.header = msg_copy.header;
> + memcpy(&ctx->msg.payload, msg_copy.u.payload, payload_size);
>
> /*
> * The host can generate a rescind message while we
> --
> 2.34.1
>

2022-09-28 13:43:34

by Wei Liu

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