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Doesn't allowing a user mode program to access kernel space memory and execute the IN and OUT instructions defeat the purpose of having CPU modes?

Questions on Linux kernel internalsHigh memory (user space) and highmem (kernel space)What mode of memory at the time linux kernel was loaded?How is user space process/thread controlled by the operating systemGet quota usage information as non-privileged userRewrite a system call in userspaceWhere are “the kernel stack”, “Frames for C run-time startup functions”, and “Frame for main()” in the memory layout of a program?What's the use of having a kernel part in the virtual memory space of Linux processes?Linux Kernel Mode Vs User ModeCan a device driver that accesses an IO device directly run in user mode?

1

When the CPU is in user mode, the CPU can't execute privileged instructions and can't access kernel space memory.

And when the CPU is in kernel mode, the CPU can execute all instructions and can access all memory.

Now in Linux, a user mode program can access all memory (using /dev/mem) and can execute the two privileged instructions IN and OUT (using iopl() I think).

So a user mode program in Linux can do most things (I think most things) that can be done in kernel mode.

Doesn't allowing a user mode program to have all this power defeats the purpose of having CPU modes?

linux

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1

When the CPU is in user mode, the CPU can't execute privileged instructions and can't access kernel space memory.

And when the CPU is in kernel mode, the CPU can execute all instructions and can access all memory.

Now in Linux, a user mode program can access all memory (using /dev/mem) and can execute the two privileged instructions IN and OUT (using iopl() I think).

So a user mode program in Linux can do most things (I think most things) that can be done in kernel mode.

Doesn't allowing a user mode program to have all this power defeats the purpose of having CPU modes?

linux

share|improve this question

edited 1 hour ago

ilkkachu

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asked 2 hours ago

user341099user341099

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user341099 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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When the CPU is in user mode, the CPU can't execute privileged instructions and can't access kernel space memory.

And when the CPU is in kernel mode, the CPU can execute all instructions and can access all memory.

Now in Linux, a user mode program can access all memory (using /dev/mem) and can execute the two privileged instructions IN and OUT (using iopl() I think).

So a user mode program in Linux can do most things (I think most things) that can be done in kernel mode.

Doesn't allowing a user mode program to have all this power defeats the purpose of having CPU modes?

linux

share|improve this question

edited 1 hour ago

ilkkachu

60.6k1098172

asked 2 hours ago

user341099user341099

61

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user341099 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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edited 1 hour ago

ilkkachu

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asked 2 hours ago

user341099user341099

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edited 1 hour ago

ilkkachu

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user341099user341099

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edited 1 hour ago

ilkkachu

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edited 1 hour ago

ilkkachu

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user341099user341099

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So a user mode program in Linux can do most things (I think most things) that can be done in kernel mode.

Well, not all use mode programs can, only those with the appropriate privileges. And that's determined by the kernel.

/dev/mem is protected by the usual filesystem access permissions, and the CAP_SYS_RAWIO capability. iopl() and ioperm() are also restricted through the same capability.

/dev/mem can also be compiled out of the kernel altogether (CONFIG_DEVMEM).

Doesn't allowing a user mode program to have all this power defeats the purpose of having CPU modes?

Well, maybe. It depends on what you want privileged user-space processes to be able to do. User-space processes can also trash the whole hard drive if they have access to /dev/sda (or equivalent), even though that defeats the purpose of having a filesystem driver to handle storage access.

(Then there's also the fact that iopl() works by utilizing the CPU privilege modes on i386, so it can't well be said to defeat their purpose.)

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edited 1 hour ago

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answered 1 hour ago

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2

So a user mode program in Linux can do most things (I think most things) that can be done in kernel mode.

Well, not all use mode programs can, only those with the appropriate privileges. And that's determined by the kernel.

/dev/mem is protected by the usual filesystem access permissions, and the CAP_SYS_RAWIO capability. iopl() and ioperm() are also restricted through the same capability.

/dev/mem can also be compiled out of the kernel altogether (CONFIG_DEVMEM).

Doesn't allowing a user mode program to have all this power defeats the purpose of having CPU modes?

Well, maybe. It depends on what you want privileged user-space processes to be able to do. User-space processes can also trash the whole hard drive if they have access to /dev/sda (or equivalent), even though that defeats the purpose of having a filesystem driver to handle storage access.

(Then there's also the fact that iopl() works by utilizing the CPU privilege modes on i386, so it can't well be said to defeat their purpose.)

share|improve this answer

edited 1 hour ago

Stephen Kitt

175k24400477

answered 1 hour ago

ilkkachuilkkachu

60.6k1098172

add a comment | 

2

So a user mode program in Linux can do most things (I think most things) that can be done in kernel mode.

Well, not all use mode programs can, only those with the appropriate privileges. And that's determined by the kernel.

/dev/mem is protected by the usual filesystem access permissions, and the CAP_SYS_RAWIO capability. iopl() and ioperm() are also restricted through the same capability.

/dev/mem can also be compiled out of the kernel altogether (CONFIG_DEVMEM).

Doesn't allowing a user mode program to have all this power defeats the purpose of having CPU modes?

Well, maybe. It depends on what you want privileged user-space processes to be able to do. User-space processes can also trash the whole hard drive if they have access to /dev/sda (or equivalent), even though that defeats the purpose of having a filesystem driver to handle storage access.

(Then there's also the fact that iopl() works by utilizing the CPU privilege modes on i386, so it can't well be said to defeat their purpose.)

share|improve this answer

edited 1 hour ago

Stephen Kitt

175k24400477

answered 1 hour ago

ilkkachuilkkachu

60.6k1098172

add a comment | 

So a user mode program in Linux can do most things (I think most things) that can be done in kernel mode.

Well, not all use mode programs can, only those with the appropriate privileges. And that's determined by the kernel.

/dev/mem is protected by the usual filesystem access permissions, and the CAP_SYS_RAWIO capability. iopl() and ioperm() are also restricted through the same capability.

/dev/mem can also be compiled out of the kernel altogether (CONFIG_DEVMEM).

Doesn't allowing a user mode program to have all this power defeats the purpose of having CPU modes?

Well, maybe. It depends on what you want privileged user-space processes to be able to do. User-space processes can also trash the whole hard drive if they have access to /dev/sda (or equivalent), even though that defeats the purpose of having a filesystem driver to handle storage access.

(Then there's also the fact that iopl() works by utilizing the CPU privilege modes on i386, so it can't well be said to defeat their purpose.)

share|improve this answer

edited 1 hour ago

Stephen Kitt

175k24400477

answered 1 hour ago

ilkkachuilkkachu

60.6k1098172

share|improve this answer

edited 1 hour ago

Stephen Kitt

175k24400477

answered 1 hour ago

ilkkachuilkkachu

60.6k1098172

edited 1 hour ago

Stephen Kitt

175k24400477

edited 1 hour ago

Stephen Kitt

175k24400477

answered 1 hour ago

ilkkachuilkkachu

60.6k1098172

answered 1 hour ago

ilkkachuilkkachu

60.6k1098172