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Date: Mon, 14 Jan 2019 17:09:52 -0800
From: Kees Cook <keescook@...omium.org>
To: Willy Tarreau <w@....eu>
Cc: Silvio Cesare <silvio.cesare@...il.com>,
LKML <linux-kernel@...r.kernel.org>,
Mark Brown <broonie@...nel.org>,
Dan Carpenter <dan.carpenter@...cle.com>,
Will Deacon <will.deacon@....com>, Greg KH <greg@...ah.com>
Subject: Re: [PATCH 8/8] spi: dw: change snprintf to scnprintf for possible overflow
On Sat, Jan 12, 2019 at 7:28 AM Willy Tarreau <w@....eu> wrote:
>
> From: Silvio Cesare <silvio.cesare@...il.com>
>
> Change snprintf to scnprintf. There are generally two cases where using
> snprintf causes problems.
>
> 1) Uses of size += snprintf(buf, SIZE - size, fmt, ...)
> In this case, if snprintf would have written more characters than what the
> buffer size (SIZE) is, then size will end up larger than SIZE. In later
> uses of snprintf, SIZE - size will result in a negative number, leading
> to problems. Note that size might already be too large by using
> size = snprintf before the code reaches a case of size += snprintf.
>
> 2) If size is ultimately used as a length parameter for a copy back to user
> space, then it will potentially allow for a buffer overflow and information
> disclosure when size is greater than SIZE. When the size is used to index
> the buffer directly, we can have memory corruption. This also means when
> size = snprintf... is used, it may also cause problems since size may become
> large. Copying to userspace is mitigated by the HARDENED_USERCOPY kernel
> configuration.
>
> The solution to these issues is to use scnprintf which returns the number of
> characters actually written to the buffer, so the size variable will never
> exceed SIZE.
>
> Signed-off-by: Silvio Cesare <silvio.cesare@...il.com>
> Cc: Mark Brown <broonie@...nel.org>
> Cc: Dan Carpenter <dan.carpenter@...cle.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Greg KH <greg@...ah.com>
> Signed-off-by: Willy Tarreau <w@....eu>
Reviewed-by: Kees Cook <keescook@...omium.org>
-Kees
>
> ---
> drivers/spi/spi-dw.c | 36 ++++++++++++++++++------------------
> 1 file changed, 18 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/spi/spi-dw.c b/drivers/spi/spi-dw.c
> index b705f2bdb8b9..008d52d37439 100644
> --- a/drivers/spi/spi-dw.c
> +++ b/drivers/spi/spi-dw.c
> @@ -54,41 +54,41 @@ static ssize_t dw_spi_show_regs(struct file *file, char __user *user_buf,
> if (!buf)
> return 0;
>
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "%s registers:\n", dev_name(&dws->master->dev));
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "=================================\n");
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "CTRL0: \t\t0x%08x\n", dw_readl(dws, DW_SPI_CTRL0));
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "CTRL1: \t\t0x%08x\n", dw_readl(dws, DW_SPI_CTRL1));
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "SSIENR: \t0x%08x\n", dw_readl(dws, DW_SPI_SSIENR));
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "SER: \t\t0x%08x\n", dw_readl(dws, DW_SPI_SER));
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "BAUDR: \t\t0x%08x\n", dw_readl(dws, DW_SPI_BAUDR));
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "TXFTLR: \t0x%08x\n", dw_readl(dws, DW_SPI_TXFLTR));
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "RXFTLR: \t0x%08x\n", dw_readl(dws, DW_SPI_RXFLTR));
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "TXFLR: \t\t0x%08x\n", dw_readl(dws, DW_SPI_TXFLR));
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "RXFLR: \t\t0x%08x\n", dw_readl(dws, DW_SPI_RXFLR));
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "SR: \t\t0x%08x\n", dw_readl(dws, DW_SPI_SR));
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "IMR: \t\t0x%08x\n", dw_readl(dws, DW_SPI_IMR));
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "ISR: \t\t0x%08x\n", dw_readl(dws, DW_SPI_ISR));
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "DMACR: \t\t0x%08x\n", dw_readl(dws, DW_SPI_DMACR));
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "DMATDLR: \t0x%08x\n", dw_readl(dws, DW_SPI_DMATDLR));
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "DMARDLR: \t0x%08x\n", dw_readl(dws, DW_SPI_DMARDLR));
> - len += snprintf(buf + len, SPI_REGS_BUFSIZE - len,
> + len += scnprintf(buf + len, SPI_REGS_BUFSIZE - len,
> "=================================\n");
>
> ret = simple_read_from_buffer(user_buf, count, ppos, buf, len);
> --
> 2.19.2
>
--
Kees Cook
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